How to make a PDF smaller
Most of a large PDF is images, and most of the rest is fonts and leftover objects. Knowing which one you have decides which setting will help.
Why is my PDF so large?
Almost always because the pages are pictures. A page of text stored as text is a few kilobytes; the same page stored as a 300 DPI color scan is one to three megabytes, and a twenty-page scan is therefore a forty-megabyte file made of twenty photographs.
The other two causes are smaller but real. Embedded fonts add a few hundred kilobytes each and are often embedded several times over after documents have been combined, and a file that has been edited repeatedly still carries earlier versions of every object it ever held, because a PDF is normally saved by appending rather than by rewriting.
Which setting actually shrinks a file?
Resolution first, encoder quality second. Resampling an image from 300 DPI to 150 DPI removes three quarters of its pixels before any compression happens at all, which is a far larger saving than any quality slider will give you at full resolution.
After that the quality setting decides the rest. Around 80 is where most people stop being able to see a difference on screen; below about 60 the flat areas of a scan start to show square blocks around the letters.
What does compression cost me?
Sharpness in the pictures, and nothing else. Text that is real text stays perfectly crisp at any setting because it is not being re-encoded; only the images are. That is also why a text document barely shrinks while a scan shrinks enormously.
The cost becomes visible on paper. A scan resampled to 100 DPI looks fine on a phone and soft when printed, so choose the resolution for the destination: 150 DPI for reading on screen, 200 to 300 if the file will be printed or read by recognition software later.
What if it is still too large?
Split it. A twelve-megabyte attachment limit is a limit on one message rather than on the document, and sending three files of four megabytes moves the same pages. That is usually better than reaching the limit by degrading the images until they are unpleasant.
If the file is mostly text and still large, the structure is the problem rather than the pictures, and rewriting the document drops the accumulated leftovers. Fonts are the one thing this suite will not shrink: it does not subset them, and no setting here pretends otherwise.
Where to go from here
The compression tool shows an estimate before you commit and the exact before and after bytes afterwards. Optimizing for the web rewrites the structure without touching the images, and exporting the pages as JPGs is the honest answer when what you wanted was pictures all along.
What DPI and quality actually change takes the two settings above apart properly, and PDF privacy on the web explains why none of this needs an upload.