Why is my PDF so large?

Find the cause in a minute

You rarely need special software to see what is making a PDF big. Try these four checks, in this order:

1. Work out the size per page. Divide the file size by the number of pages. A megabyte or more per page points to scans or photos. A few dozen kilobytes per page is normal for typed text.
2. Try to select a word. Press and hold on a phone, or drag across it with a mouse. If nothing gets selected, the page is a picture. Our PDF Viewer tells you the same thing: searching a scanned page finds no text.
3. Zoom in a long way. Real text stays crisp at any zoom. Text inside a picture turns blocky or blurry.
4. Compress a copy as a test. Put the file through the compressor at Recommended. A big drop means the weight was in pictures stored sharper than needed. If it says "Already compact", pictures are not the problem, so look at fonts, the number of pages, or hidden data.

Then match what you saw to a cause:

To find the heaviest pages, cut a copy into single pages with Split PDF (choose Every page) and sort the files by size. The parts add up to more than the whole, because each one carries its own copy of shared fonts and pictures: our 117-page, 5.07 MB textbook became 117 PDFs totalling 29.4 MB. But the heavy pages stand out at once.

Cause 1: scans made at high resolution, in colour

This is the most common cause by far. A scanner saves each page as a picture, and office scanners and copiers often scan at 300 dpi (dots per inch) or more, in colour. A colour A4 page at 300 dpi is about 8.7 million pixels, and in our test files each scanned page took 1 to 2.5 MB. Scanning at 600 dpi makes four times as many pixels as 300 dpi, because the count doubles both across and down the page.

Fix it now: compress it. Scans are where the compressor does best, because it scales oversized pictures down to what the page needs. Our sample file, a 4-page colour scan at 300 ppi, went from 9.96 MB to 563 KB at Recommended, and a 4-page grayscale scan from 6.76 MB to 0.98 MB.

Scan smaller next time:

• Pick the resolution for the job. 150 dpi is plenty for reading on a screen. Use 200 to 300 dpi if the paper will be printed or has very small print. Higher settings make huge files without making text easier to read.
• Use grayscale for black-and-white papers. Letters, forms, receipts and marksheets rarely need colour, and grayscale scans are much smaller.
• Be careful with pure black and white. Some scanners offer a black-and-white mode that makes tiny files. It suits clean typed pages, but faint stamps, pencil marks and photos can vanish or turn harsh. Grayscale is the safer choice.
• Set the paper size. If the scanner scans its whole glass, you store a grey border around every page. Choosing A4 in the scan settings avoids it.

Cause 2: photos from a phone camera

Phone cameras make big pictures. A photo from a 12-megapixel camera has 12 million pixels and usually takes a few megabytes. A good converter puts photos into a PDF untouched, so the PDF is as big as the photos. In our tests, JPG to PDF turned 20 photos of 12 megapixels each, 62.4 MB in all, into a 62.4 MB PDF. That is the right behaviour for quality, but not for sending.

On an A4 page, a 12-megapixel photo works out to about 360 pixels per inch, more than twice the 150 that look sharp on screens and office printers.

Fix it now: compress the PDF after making it. The compressor brings each photo down to the level you choose. If a form wants an exact size, like 200 KB, use Resize PDF instead.

Make it smaller next time:

• Use a document scanner on your phone. Apps you may already have can do it, like Google Drive on Android and Notes or Files on iPhone. They crop to the page, straighten it and save a PDF, usually much smaller than a raw photo.
• Fill the frame with the page. Table, floor and fingers around the paper are pixels too.
• Use good, even light. Shadows and grain from a dim room add detail that takes room and helps nobody.

Cause 3: pictures pasted at full size

When you put a photo into a Word document or a slide and drag it smaller, the app usually still keeps the whole photo. It looks like a stamp on the page, but it carries every pixel of the original, and so does the PDF you save from it. Cropping works the same way: in many apps, the cut-off part of a photo is only hidden, not removed.

That is how a one-page certificate with a logo and a passport-size photo can come out at several megabytes.

Fix it now: compress it. The compressor measures how big each picture is actually drawn on the page, so a full-size photo shown as a stamp is scaled down to what that stamp needs.

Make it smaller next time: Microsoft Office apps have a Compress Pictures command that lowers the resolution of pictures in the document and can delete the cropped-off parts. Run it before you save as PDF. Or shrink big photos before you insert them.

Cause 4: exports from design and presentation apps

Design apps are made for print and slide apps for big screens, so the PDFs they export can be heavy:

• Print settings. A "print" export keeps every picture at full resolution, and may add extra space around the page for trimming, called bleed.
• Full-page photos. A slide deck with a photo behind every slide carries one big picture per page.
• Editing data. Some design programs, Illustrator and Photoshop among them, can store a complete editable copy of the artwork inside the PDF, so it can be opened and changed later. None of it shows on the page, but all of it takes space.

Fix it now: compress it. Oversized pictures are resized, and hidden page previews and editing leftovers are dropped. The pages look the same.

Export smaller next time:

• In a design app, pick the standard or screen option instead of the print one when the PDF is for sharing. Adobe's design apps have a preset named for the smallest file size.
• In Word and PowerPoint on Windows, choose the smaller, minimum-size option when you save as PDF.
• Turn off the option that keeps editing capabilities for the copy you send, and keep your editable original separately.
• Keep the print-quality export for the print shop, and send everyone else a smaller copy.

Cause 5: embedded fonts

So that text looks exactly right on any device, a PDF usually stores the fonts it uses inside the file. This is called embedding. Normally only the letters actually used are stored, which is called a subset and takes very little room. Fonts become the weight when:

• a document uses many different fonts, especially complete copies of each
• a font covers Chinese, Japanese or Korean, which have thousands of characters, so a complete copy can be several megabytes on its own
• many small PDFs are merged, and each one brings its own fonts

How to check: Adobe Acrobat Reader's document properties window has a Fonts tab that lists every font in the file. "Embedded Subset" next to a name means only the letters used are stored. A font listed as embedded without the word subset is stored whole.

The honest fix: our compressor keeps fonts as they are, because the text needs them to show correctly, so it can't fix this cause. What helps is making the PDF again from the original document with fewer fonts, using the app's standard PDF export. Text-heavy files simply have a smallest size: our 14-page research paper, mostly text, went from 992 KB to 766 KB, a 23% drop.

Cause 6: the same picture stored again and again

Letterheads, logos, watermarks and page backgrounds appear on every page. Some programs store that picture once and reuse it. Others store it again for every page, so a 50-page report with a full-page background can carry 50 copies of the same picture.

Merging does this too. Merge PDF copies every page with its own pictures and fonts, so ten letters with the same letterhead, merged, carry ten copies of the letterhead.

Fix it now: compress it. The compressor stores repeated pictures once, so all the pages point to a single copy. Compressing after merging is a good habit for exactly this reason.

Next time: put the logo in the document's header or on the master slide instead of pasting it onto each page. Apps usually store it once that way.

Cause 7: pages saved as pictures

Sometimes a document that started life as text ends up as pictures of text. It happens when someone:

• takes a screenshot of each page and makes a PDF from the screenshots
• prints a digital document and scans the printout, or photographs the screen
• turns PDF pages into JPGs and then back into a PDF
• uses a print option that sends each page as an image

The result is bigger, softer and no longer searchable. A typed page that needed a few kilobytes now needs hundreds.

Fix it now: compressing still shrinks the pictures, often a lot. But they stay pictures. Our tools don't recognise text in pictures (that is called OCR), so the text won't become searchable again.

The better fix: go back to the digital original. Download the statement again from net banking instead of scanning the printout. Ask the sender for the PDF they exported, not a scan of it. When you make a PDF yourself, use Save as PDF in the app, or Save as PDF in your browser's print window, which keeps text as text.

Cause 8: hidden extras

Some of a PDF's weight never shows on any page:

• Old versions. Many PDF editors save a change by adding it to the end of the file, keeping the old version too. After many rounds of edits, comments or form filling, the file can hold several versions of the same pages.
• Page previews. Some programs store a small preview picture of every page.
• Editing data from design apps, as in cause 4.
• Loosely packed data. Some programs save text and drawings with weak compression, or none at all.

Fix it now: compress it, even at Best quality. As the compressor's guide explains, old versions and data that nothing uses are removed, previews and editing leftovers are dropped, and everything else is packed tightly. If the file is mostly text, the drop may be modest, but the copy you send carries no leftovers from earlier drafts.

When a PDF is big for a good reason

Some PDFs are meant to be big, and making them small would spoil them:

• Print-ready files for a print shop: brochures, posters, wedding cards and photo books. Send the printer the full-quality file.
• Detailed drawings and maps, made of thousands of lines. Lines are not pictures, so they stay as they are, and these files shrink less.
• Long scanned books with hundreds of pages. Even at a sensible size per page, the total adds up.

For files like these, keep the original for printing and make a smaller copy for sharing. For a long document, send only the chapters people need, cut out with Split PDF, or share a link instead of an attachment.

For everything else, the fix is usually one step: compress the PDF and check the result. How to reduce PDF file size explains the levels and what results to expect, and compressing a PDF for email covers attachment limits.