Most email services bounce attachments over about 20-25 MB, and a PDF full of scans or photos blows past that fast. Compressing re-encodes the images inside the file so it fits, while leaving the text crisp — no printing, no screenshotting.
Just fit it under a size
If you have a hard limit — say 10 MB for a portal upload — use the target-size option instead of guessing a quality level. Enter the number and the tool works down a quality ladder until the result fits, so you get the best-looking file that still clears the limit.
Why did nothing change?
Text-only PDFs are already about as small as they get, so there's little to squeeze — if our result would be bigger than your original, we hand back the original untouched. Compression pays off most on scanned documents and image-heavy files.