Why some PDFs shrink and others do not
Compressing a PDF is not one operation applied harder or softer. It is mostly one thing: re-saving the photographs inside the file at a lower resolution. That explains both of the questions people bring here — which setting to pick, and why a thirty-megabyte document sometimes comes back at twenty-nine.
Which tool you need
A file with photographs or scanned pages in it
Compress PDF will help, often a lot. Each option shows the size it reaches before you run it, so you can pick by the number rather than by the name.
Quality that must not change at all
Choose the best-quality option, which changes no picture. It saves less, and what it does save costs nothing visible.
A text-heavy or form-heavy document that barely moved
That is the tool working correctly and your file having little to give. Where nothing can be saved, your original is kept rather than a slightly larger rebuild handed back.
A file that has to fit an email limit whatever it costs
Send less rather than compress harder: take out the pages that matter, or split the document and send it in parts. Compress will not turn your pages into pictures to hit a number.
Only part of the document needs sending
Extract PDF pages first. Three pages of a long report is a far bigger saving than any compression setting, and the pages keep their text.
What each option is doing
Best quality changes no picture at all. The other options re-save the photographs at a lower resolution, which is where the saving comes from, and the size each one reaches is shown before you run it so the choice is made on the number rather than on a word.
Text and vector artwork are never rebuilt on any of them, and no option turns a page into a picture, so the text stays selectable and searchable whichever you pick.
When nothing helps
A PDF that will not shrink is usually one whose size is not in its pictures. Embedded fonts, a document built from a huge vector drawing, and pages that are already compressed as well as they can be all look the same from outside: a large file with nothing to reclaim.
The way out is usually to send less rather than to compress harder. Pull out the pages that matter, or split the document up. Where the whole thing genuinely has to travel as one file, flattening is the last option and its cost is the text.
What to know before you start
- Compressing cannot reach a size you name. You choose an option, see what it reaches, and decide.
- A locked document has to be unlocked first, with its password.
- Nothing is uploaded; the file is made smaller in your browser, and the original is left as it was.