Why a PDF gets big, and how to make it smaller

To make a PDF smaller, shrink the pictures inside it. Photos and scanned pages are usually most of a big PDF's size, and a compressor lowers their resolution and saves them again as JPEG, while the pages, the text and the fonts stay. In our test, three pages of photos went from 11.7 MB to 201 KB.

Every number in this guide comes from test files we made ourselves and ran through our own PDF size reducer on 22 October 2026.

What makes a PDF big: pictures, embedded fonts and scans

Text on its own takes very little room. Nine pages of plain text came to 51 KB in our tests, about 6 KB a page. When a PDF runs to megabytes, the space has almost always gone on pictures, on fonts saved inside the file, or on scanned pages.

Pictures

A picture in a PDF keeps every pixel it had, however small it looks on the page. Our photo test file has three A4 pages, each with a heading and a photo about 4,000 pixels wide set across most of the page. That works out at about 565 dots per inch (DPI) on paper, and the file came to 11.7 MB. The photos were all of it except a few kilobytes.

Fonts

A PDF carries its fonts with it, so it looks the same on any device: the Library of Congress, describing the PDF format, notes that faithful rendering requires that fonts be embedded. An app can save only the letters the document uses (a font subset) or the whole font file. Two pages of text with a whole font inside came to 408 KB, eight times the size of our nine pages saved with only the letters they use.

Scans

A scanned page is a photo of the paper, so the words on it are part of the picture. We drew three pages of text at 300 DPI, as a scanner does, with a faint paper tint and grain. In colour they came to 3.8 MB, about 1.3 MB a page. In black and white the same pages took 159 KB, about 24 times less.

Black and white wins because a picture with only two shades can be stored with a fax method made for scanned paper, which the Library of Congress describes as lossless compression of bi-level images. For a page of plain black text, a black-and-white scan loses nothing you need.

What compression can and cannot remove

Compress PDF works on those same parts. It keeps every page and all the text at every level, and checks each result before handing it back. Here is what it did to our test files:

What compression removes

• Extra pixels. Recommended brings pictures down to 150 DPI and High to 110 DPI, so our photos went from about 565 DPI to 150. A black-and-white scan can end a little lower: ours came out at 100 DPI at High.
• Fine detail. Colour and grey pictures are saved again as JPEG, at quality 80 on Recommended and 60 on High.
• Unused letters. Recommended and High keep only the letters a font needs, which took the whole-font file from 408 KB to 39 KB.
• Slack in the file itself. Low rebuilds the structure of the PDF and packs it tighter without touching a picture, which took the plain text file from 51 KB to 31 KB.

What compression cannot remove

• Pages and text. Every page and every word stay, by design, so a long PDF is still long and a text-only one has little more to give.
• Files attached inside the PDF. Our one-page PDF carrying a 2 MB file stayed at 2.0 MB at every level, with the file still attached. Asked for 500 KB, Compress PDF sent back the smallest version it could make, with a note that the target was out of reach.
• Detail that is already gone. Compressing our High result a second time saved less than 1%, and nothing removed in an earlier pass comes back.
• The words in a scan. They are part of the picture, so compressing cannot make them searchable, and at High they soften along with everything else. Check the small print before you send it.

Three ways to make a PDF smaller

Compress it at Recommended or High

For most PDFs, Recommended is the place to start. Low is for a file that must look exactly the same, such as a print proof, and it saves little when the size is in the pictures: under 1 KB of our 11.7 MB photo file. High gives the smallest file and suits a PDF that will only be read on screen. If no smaller version passes the checks, you get your original back unchanged.

Compress PDF runs on our server, so the PDF is uploaded, and it is deleted with its smaller copy after 24 hours. The free plan takes PDFs of up to 25 MB, 3 files a day, conversions included.

Aim for a size limit

When a form or an email sets a limit, choose Target size and give the size in KB, where a KB is 1,000 bytes. The pictures are lowered step by step, down to 72 DPI at most, until the PDF fits: our 3.8 MB colour scan, asked for 300 KB, came back at 257 KB. Look up the email and upload size limits of the place the file is going, and aim a little under.

Rebuild it from smaller pictures

If you made the PDF from photos yourself, you can do better than squeezing it afterwards. Compress the pictures first: switch on Also make it smaller in pixels and set a Longest side, such as 1920, and you can compare each photo with the original before you save it.

Then turn the pictures into one PDF. The PDF now holds no more pixels than the photos you chose. With Keep JPGs exactly as they are switched off, JPG to PDF also saves every page again as a JPEG at Smaller or Balanced quality, but it never makes a picture smaller in pixels, which is why resizing first helps.

When to split or re-export instead

Compression is not always the right tool. Something else works better when:

• Only one page is needed. A certificate or a signed page can go on its own: convert PDF pages to JPG and send just that picture. One page of our colour scan came to 280 KB as a 150 DPI JPG at Balanced.
• The PDF is mostly text. Turning text into pictures makes it bigger, not smaller: our nine pages of text, 51 KB as a PDF, came to 4.1 MB as 150 DPI JPGs. Keep text as text.
• The file is long and the limit is per email. Make two smaller PDFs from the original photos or document and send them separately, or share a link to the file instead.
• You still have the original. A fresh PDF made from the source document, or from photos you have resized, starts clean, while compressing a PDF that was compressed before gains little.

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