Why won't my PDF get smaller?
Compressing a PDF sometimes barely changes it. Here is what actually makes a PDF large and what to do when an upload limit won’t budge.
The upload form says 5MB, your PDF is 18MB and the compressor you tried got it to 17. That isn’t a broken compressor. It is a sign that the size is somewhere compressors don’t look.
What makes a PDF large
Photographs and scans. Nearly always the biggest part. A phone photo is several megabytes and a scanner set to colour at high resolution produces a large image for every page. A PDF built from either is mostly pictures.
Embedded fonts. A PDF carries the fonts it uses so it looks the same everywhere. Usually only the characters used are included, but some programs embed whole fonts and a document with several typefaces, especially ones covering Asian scripts, can carry megabytes of them.
Leftovers. Some programs save edits by adding to the end of the file instead of rewriting it, so a PDF edited many times can carry old versions of its contents. Attachments, embedded thumbnails and form data also add up.
What a compressor can do
A good compressor goes after the photographs: it reduces their resolution to something sensible and saves them again as efficient JPEGs. That is what Compress PDF does and it leaves the text, fonts and vector drawings exactly as they were, so the document is still searchable and sharp when zoomed.
Many online compressors take a shortcut instead and turn every page into a picture. The file does shrink, but the text is no longer text: nothing can be searched or copied and screen readers can’t read it.
If a PDF has no photos, no honest compressor can do much and a good one will tell you so.
When you still need to hit a limit
- Remove pages you don’t need. Cover sheets, blank pages and appendices are pure weight. Delete pages before compressing.
- Split it. Some portals accept several files. Split the PDF into parts under the limit.
- Rebuild from smaller images. If the PDF is a set of phone photos, compress the photos first and then make the PDF with image to PDF.
- Rescan sensibly. Greyscale at 200 to 300 DPI is plenty for most documents and is a fraction of the size of colour at 600.
The same thinking applies to a single photo that is too big to send, covered in your photo is too large to email.
For a related next step see Prepare documents for an online application.
Common questions
Why did compressing make almost no difference?
Because the size wasn’t in the images. A PDF that is mostly text is already small and its bulk is usually embedded fonts, which compression tools leave alone so the document still displays correctly.
Can I make a text PDF smaller?
Sometimes. Removing pages you don’t need is the most reliable way. Re-exporting from the original program with a setting such as smallest file size or without embedding full fonts, also helps. Printing to PDF again can shrink a file, but can also lose links and selectable text.
Why is a scanned PDF so big?
Every page is a photograph. A page scanned in colour at 600 DPI is a large image and a 20-page scan is 20 of them. Compressing the images or rescanning in greyscale at 200 to 300 DPI, makes the biggest difference.
Does compressing a PDF reduce quality?
It reduces the resolution and quality of the photographs inside it. At a light setting this isn’t visible on screen. Text and vector graphics aren’t affected when the compressor only touches images.