Every setting it tries is a real compression of your file, measured. What you save is a copy
you have already looked at, page by page.
A size limit, not a guess
Type the limit the upload form gives, or pick 200 KB, 500 KB, 1 MB or 2 MB. Several settings are tried on your actual file, each one is measured, and the sharpest one under the limit is the copy you get.
Text stays text
Keep text makes the pictures inside the PDF smaller and leaves everything else alone. Fonts, links, bookmarks and form fields come through unchanged, and the text can still be selected, copied and searched.
Pages as pictures, when nothing else fits
Every page becomes one JPEG. It goes much further on drawings and stubborn scans, and the tool is plain about what that costs: text that can no longer be selected, and no links or bookmarks.
Check the small print first
Switch every page between Original and Compressed, open one large, zoom in, and press Space to flip between the two. You see what the person receiving it will see, before the file exists.
It tells you when it cannot
If a limit cannot be met, you still get the smallest copy it could make, marked as over the limit, with the next thing to try. If a file would not get smaller at all, there is nothing to save, and it says so.
Careful with what it does not understand
CMYK pictures, transparency masks and rarer image formats are copied across untouched rather than guessed at. A few kilobytes left on the table is a better deal than a colour shift in your document.
