Reduce PDF file size without losing quality

The short answer

To reduce a PDF's file size, make the pictures inside it smaller and leave the text alone. That is exactly what the NoUploadPDF compressor does, inside your browser, on your phone or computer:

1. Open the compressor and tap or click Choose PDF files.
2. Leave the level on Recommended. It keeps pictures sharp on screens and office printers.
3. Download the smaller copy. Its name ends in -compressed.pdf, and your original stays as it was.
4. Open it and zoom in on the smallest print before you send it. The checklist below says what to look for.

How much smaller it gets depends on what your PDF is made of. In our tests at Recommended, scanned documents shrank by 77 to 94%, while a 14-page research paper that was mostly text shrank by 23%. The rest of this article explains why, and what to do when the usual answer isn't enough.

What actually makes a PDF big

Pictures. A page of typed words needs only a few kilobytes, because the PDF stores the letters themselves plus a font to draw them with. A picture is stored as millions of tiny coloured dots, called pixels, and every one of them takes room.

Here is the scale of it. A colour A4 page scanned at 300 dpi is about 8.7 million pixels. In our test files, each scanned page took 1 to 2.5 MB. The same page typed in a word processor and saved as a PDF would be a small fraction of that.

So when a PDF is big, look at what is on its pages:

• Scanned pages or photos of paper. Every page is one big picture. These files shrink the most.
• Typed pages with photos, charts or screenshots. The text is light and the pictures are the weight. These shrink a lot or a little, depending on how sharply the pictures were stored.
• Typed pages with no pictures. Already compact. Most of the file is text, fonts and the lines that make up tables. These shrink least.

A quick test for your own file: divide its size by its number of pages. Around a megabyte or more per page almost always means scans or photos. Under a hundred kilobytes per page usually means mostly typed text. A second test: try to select a word with your finger or mouse. If you can't, that page is a picture.

Less common causes, like a logo stored again on every page, are listed in the compressor's guide, and Why is my PDF so large? shows how to spot and fix each one.

How compression treats pictures and text

Good compression treats the two very differently. That is the whole secret of a smaller PDF that doesn't look worse.

Text, fonts and drawings: packed tighter, never changed

Letters, lines, tables and vector drawings (shapes stored as instructions, like "draw a line from here to there") are packed more tightly with lossless compression. Lossless means nothing changes: like a ZIP file, every letter comes back exactly as it was. The text stays crisp at any zoom, and you can still search and copy it.

Pictures: resized to what the page needs

Pictures are where the savings are. The compressor works out how big each picture is actually drawn on the page, and how many pixels it has for each inch of paper. That measure is called ppi, pixels per inch. A page scanned at 300 dpi holds 300 pixels for every inch. That is far more than you need: on a screen or an office printer, 150 per inch already looks sharp. So the compressor scales the picture down to your level's limit and saves it again as JPEG, the usual format for photos, at a matching quality.

Charts, logos and screenshots with few colours are handled differently. JPEG would leave faint smudges around their sharp edges, so they are kept with lossless compression instead.

What "without losing quality" really means

Be careful with any promise of a much smaller PDF with no change at all. Lossless packing changes nothing you can see, but on a scan it saves little, because the pictures inside are often JPEGs already. The big savings come from dropping detail nobody will see.

Done well, that loss is invisible. At 150 ppi, a picture that fills the width of an A4 page is still about 1,240 pixels wide, which is wider than many phone screens. Done badly, it blurs scanned text or turns typed pages into pictures. The honest promise is this: text never loses anything, and pictures lose only the detail above the level you chose.

Choosing between the three levels

Choose the level by asking who will look at the file, and how. The level only decides how sharp pictures stay: up to 220 ppi at Best quality, 150 at Recommended and 100 at Smallest. Text looks the same at every level. The level table in the guide has the short version; here is how it works out in real situations.

Not sure? Try two levels. Your original is never changed, so you can compress the same file twice and keep the copy you like. The level is read when you add a file, so pick the new level first, then choose the same PDF again. Both copies are named -compressed.pdf, and your browser usually adds a number to the second one, so note which is which.

Always start from the original. Compressing an already compressed copy loses a little more picture detail each round, and saves less each time.

What results to expect

These are real files we compressed at Recommended in Google Chrome on our test computer, grouped by kind so you can find the one most like yours.

Each of these took between 0.3 and 1.3 seconds. Bigger files take longer: in Chrome on a desktop computer, a 477 MB scan took 106 seconds and a 1.1 GB scan 266 seconds, still without uploading anything. Across our test set of 89 PDFs, all 86 that could be opened were compressed correctly; the other three were a password-protected file and two damaged ones.

Your own result will differ, but the pattern holds: the more of your PDF is pictures, the more it shrinks.

When compression barely helps, and what to do then

If your PDF is mostly typed text, compression can only do so much. Sometimes the compressor says "Already compact, so there is nothing to gain. Keep your original file." That isn't a failure: when a new copy would not be smaller, you keep your original instead of a pointless one.

What to do next depends on why you wanted a smaller file:

• Check that it is really too big. A text PDF of a few hundred kilobytes is fine for email and most upload forms. Look at the actual limit before you work hard on it.
• Send fewer pages. If a form wants only your offer letter and not the whole 30-page contract, take out the pages you need with Split PDF, or delete blank and unneeded pages with Organize PDF. Each page you leave out takes its share of the size with it.
• Send it in parts. When a limit applies to each file, such as an email limit, a long report can go as two or three PDFs. Split PDF cuts by page ranges, not by size, so check each part's size afterwards.
• Make the PDF again from the source. If you created it yourself in Word, PowerPoint or a design app, the export settings decide much of the size. Office apps can save a PDF at a smaller size meant for screens, and design apps often have a separate print-quality export that a shared copy doesn't need.
• Think about the fonts. A document that stores many complete fonts, or a font for Chinese, Japanese or Korean, can carry megabytes of font data. The compressor keeps fonts, because the text needs them to show correctly, so using fewer fonts in the original is the only fix.
• Share a link instead. For something really long, like a whole book, a link from your cloud storage is often the right answer.

Resize PDF can't take a text-heavy file lower either, because it keeps text in exactly the same way. That is why our 117-page textbook could not go below 2.56 MB when we asked it for 1 MB.

Doing it on a phone or a computer

The compressor works the same way on both, inside the browser, with no app to install. What differs is where your file is to begin with, and where the smaller copy ends up.

On an Android phone

1. Make sure the PDF is saved on the phone. If it is an email attachment, download it first. If it came in a chat app and the file picker can't find it, open it in the chat and save or share a copy to your files.
2. Open the compressor in Chrome, Samsung Internet or another up-to-date browser, pick a level and tap Choose PDF files.
3. When it is done, tap Download. The copy goes to your Downloads folder.

On an iPhone or iPad

1. Save the PDF to the Files app first. From Mail or a chat, open the PDF, tap the share button and choose Save to Files.
2. Open the compressor in Safari or Chrome, tap Choose PDF files and pick the PDF from Files.
3. Tap Download. The smaller copy usually goes to Downloads in the Files app.

On a computer

Drop one PDF or many onto the page. They are compressed one after another, and each gets its own Download button. A computer is also the better choice for very large files. While it works, the engine needs about twice the file's size in memory, and phones have less memory than computers. In our tests, files up to 50 MB needed under 200 MB of memory. For files over a few hundred megabytes, use a computer.

On any device, once the page has loaded you can switch off Wi-Fi and mobile data and keep compressing. Nothing is uploaded, so a slow connection doesn't slow you down.

Checking the result before you send it

A minute of checking saves an awkward "I can't read page 3" reply. Open the compressed copy, not the original, and go through this list:

• Zoom in on the smallest print. Roll numbers, dates, amounts, account numbers and the writing on stamps go soft first. If you have to squint, compress the original again at Recommended or Best quality.
• Look at photos and signatures. On ID cards and forms, the face photo and the signature are what a reviewer checks most closely.
• Count the pages. They should all be there, in the same order. The engine reads every new file back to check this, for files up to 600 MB, but your own eyes are the final check.
• Search for a word. In a typed document, the text should still be searchable. Our PDF Viewer has a search button that shows this quickly. Scans never had searchable text, and compressing doesn't change that.
• Check the size where it counts. Look at the size in your file manager or in the upload form. Apps count differently: some take 1 KB as 1,000 bytes and others as 1,024, so the same file can show slightly different numbers.
• Attach the right file. Make sure you pick the one ending in -compressed.pdf, not the original. If the receiver needs to know what it is, rename it to something clear, like Anita-Rao-bank-statement.pdf.

Two things change on purpose. A PDF with a digital signature loses the signature's validity when it is compressed, so keep the signed original for anyone who needs to verify it. And a PDF that had editing restrictions, but no password to open it, comes out without those restrictions. PDFs that need a password to open can't be compressed yet: remove the password in your PDF reader first.

Other ways to reduce PDF size, fairly compared

You may already have something that can shrink a PDF. Here is when each option makes sense.

Preview on a Mac

Preview can export a PDF with a filter that reduces its file size. It is built in and quick. It gives you no choice of how much to shrink, though, and scans can come out softer than you'd like, so check the small print.

Word, PowerPoint and other office apps

If you made the document yourself, shrinking its pictures inside the app, or choosing a smaller size when you save as PDF, often gives the cleanest result, because the pictures are made smaller before the PDF exists.

Adobe Acrobat

The paid version of Acrobat has detailed settings for reducing file size, and it is a solid choice if you already pay for it.

Online compressors that upload your file

Many are well made. But they send your file to someone else's server, and free plans of upload-based tools often cap the file size or the number of tasks. For bank statements, ID cards and medical reports, that upload is the step worth avoiding.

Printing to a new PDF

Not a reliable way to shrink a file, and the new copy can lose its links, bookmarks and fillable fields.

Where NoUploadPDF fits: free, no upload, works on phones and computers, and shrinks pictures to the level you choose while leaving text alone. If that suits your file, open the compressor. For a file headed to an inbox, compressing a PDF for email covers attachment limits and sending several files.