Start with the upload limit
A PDF compressor is most useful when it gives you a clear pass or fail against the file size your form requires.
Guide
Guides for smaller PDFs.
Scanned PDFs behave differently
A scanned document is often a stack of page images. Downsampling those page images can save a lot of space, but very strict targets reduce visual detail.
Text-heavy PDFs may already be small
Documents created from Word, Google Docs, or Pages often contain selectable text. A text-preserving rewrite may save some space, but images usually drive large reductions.