Optimise · 1 tool
Make a document lighter
Almost all the weight in a large PDF is photographs, so those are what get re-encoded. The text, the fonts and the drawings are left exactly as they were.
The tool
Where the weight actually is
A page of text is a few kilobytes: some glyph codes, some positions, and a font subset shared across the whole document. A photograph on that same page can be two megabytes on its own. So the only compression worth doing is on the images, and the only question worth asking is whether re-encoding them saves enough to be worth a second copy of your file.
Why the pages are not turned into pictures
Rasterising every page is how most “compress PDF” tools get their impressive numbers, and it destroys selectable text, screen-reader access and print quality to get them. That is a different operation with different consequences, and on this site it is called redaction — where losing the text is the entire point and is stated as the cost.
When the honest answer is no file
If the document holds no images, or its images are already efficiently stored, or the saving comes to under one per cent, or under ten kilobytes, or the rebuilt file is larger than the original, you are offered nothing and told which of those happened. A true number can still be a lie about the outcome: “2% smaller — 2 KB became 2 KB” is arithmetic, not a result.
What is not here yet
Repairing a damaged file, linearising for fast web view, downsampling images to a chosen resolution and OCR all belong in this group. One tool in a group of one is not a mistake to be padded out; the rest arrive when they work.