What is PPI, and why it decides how sharp your scan looks

The quick answer

PPI means pixels per inch: how many tiny dots of picture fit across one inch of the page. A scanned page is a picture made of such dots. The more dots per inch, the finer the detail and the sharper the text, and also the bigger the file. The fewer, the softer the page and the smaller the file.

When a form says "under 200 KB", something has to give. For a scanned PDF, what gives is the number of dots per inch. A typical office scanner makes 300 ppi, and a small file needs far fewer. Our tool picks the highest ppi that still fits your limit, and describes the result in one word:

• Sharp: 150 ppi or more.
• Good: 100 ppi or more.
• Readable: 45 ppi or more.
• Rough: anything below 45 ppi.

The rest of this post shows what those numbers mean in pixels, what they looked like in our tests, and what you can do about them.

Pixels on a sheet of A4

An A4 page is about 8.27 inches wide and 11.69 inches tall. Multiply by the ppi and you get the size of the picture in pixels. The 72, 150 and 300 rows below are the sizes our PDF to JPG page gives for A4. The others are worked out the same way:

See how fast the pixel count falls. Going from 300 ppi to 150 does not halve the picture: it cuts the pixels to a quarter, because the page gets half as many dots across and half as many down. That is why even a modest drop in ppi saves a lot of size. And it is why, at 45 ppi, a page has about one forty-fifth of the pixels it had at 300.

PPI, DPI and megapixels: do they differ?

DPI means dots per inch. Scanners and printers use it, because they work with dots of ink or light. PPI means pixels per inch, and screens and picture files use it. For a scan on its way into a PDF, both numbers describe the same thing: how much detail each inch of paper got. The settings of your scanner or scanner app might say 300 dpi, and our tool reports the ppi that is left after it has made the file small enough.

Megapixels count all the pixels of a picture, not per inch. A 12-megapixel phone photo has about 12 million. If such a photo fills the frame with a sheet of A4, the arithmetic gives roughly 350 ppi: far more than a form's file size can hold. That is why the tool brings the detail down, and why a photo at a normal camera setting is plenty. The post Photo to PDF under 100 KB shows how to go from phone photos to a small PDF.

One more number you may meet is JPG quality, which runs from 1 to 100. It says how hard a picture is squeezed, not how many dots it has. This post, and our result words, are about ppi.

What each word looked like in our tests

Numbers only go so far, so we opened our results and compared them with the original scan, page by page, at normal screen zoom. The sample is a colour contract with a heading, numbered paragraphs of ordinary print, a colour picture, a blue signature and a small "Page 1 of 4" line. Here is what we saw:

Notice that "Readable" covers a wide band, from 45 to 99 ppi. Near the top, the page looks almost untouched. Near the bottom, ordinary print is fine but small print struggles. So when the result says Readable, open the file and check the smallest text that matters to you: a roll number, a date, a seal.

Why small print and thin ink go first

A letter is drawn with a handful of dots. A big heading has many dots across each stroke, so it survives when the dots get fewer. A small footnote has only a few dots per stroke, and dropping the ppi leaves it with too few to form a clear letter. Thin lines and light colours behave the same way: a thin blue signature, a faint stamp or a light pen mark fade before a dark, bold one.

This shows up in our tests as a pattern:

• Big headings stayed readable longest, even at 38 ppi.
• Small print struggled first. At 48 ppi, the contract's smaller print was hard to make out.
• A blue signature stayed blue on one page and on four, and turned grey on 14 pages.
• Pages packed with small text kept fewer ppi than pages with wide margins, at the same page count.

The practical lesson: sign with a dark pen, make sure the stamp is clear on the paper before you scan, and use a short file when the signature matters.

More pages, fewer ppi

A size limit is shared by every page of the file. So the ppi a page keeps depends on how many pages share the limit. We measured it on a colour contract at 300 dpi:

Two things follow. Doubling the limit buys a clear step up. And two pages in 200 KB keep almost exactly the detail of one page in 100 KB, as four pages in 200 KB match two pages in 100 KB: what decides how a page looks is the room each page gets. The twice the room section of the 200 KB guide has the full comparison.

What you can change, and what you can't

You can't raise a scan's ppi after the fact. A scan has only the detail the scanner or camera caught, and no setting can add more. What you can control:

• The number of pages. The biggest lever. Leave out blank backs, covers and instruction pages.
• The size limit, if the form lets you choose. Twice the room means clearly more ppi.
• The crop. A page photographed on a table carries the table, and the table uses part of the room. Crop to the paper.
• Separate files, if the form has separate boxes. Each file gets its own limit.
• Your starting file. Always begin from your first scan. A file already squeezed for another form has lost detail for good.

Colour or grayscale? We tested whether grayscale helps. The four-page colour contract came out at 183 KB, Readable at 99 ppi. A grayscale copy came out at 187 KB, Good at 102 ppi: a gain of less than 3% in ppi, just enough to change the word, but not enough to see on screen. And grayscale turns a blue signature grey. So scan in colour unless the form says otherwise.

Scanner setting. Our test scans were 300 dpi, and the compressor brings the ppi down to fit, so a higher setting only makes the original file bigger.

Typed PDFs don't have a ppi

PPI only describes pictures. A PDF made on a computer holds its words as real text, drawn from fonts at any size, so the text is sharp however far you zoom. Only the pictures inside a typed PDF, such as a logo or a photo, have a ppi. If your file is typed and "won't shrink", the cause is different: Typed PDF over 200 KB? Why it won't shrink and how to fix explains it.

A quick way to tell: open the file and try to select a word with your finger or mouse. If you can, it is typed. If the page behaves like one picture, it is a scan, and ppi applies.

Your scans stay on your device

Scans are often your most private papers. Compress PDF to 200 KB has no upload step and never sends your file anywhere. It runs inside your browser, on your phone or computer. The page contacts only this site and Google's ad service, which shows the ads that keep the tools free, and our code never gives your files to the ad code. The privacy policy has the details.