Scanned PDFs
A scanned PDF stores each page as an image. Strong compression can make those page images smaller and still keep the document easy to upload.
Guide
Scanned PDFs and text PDFs compress differently.
Text PDFs
A text PDF stores letters, layout, fonts, and sometimes images. It may be searchable and selectable, but there may be less image data to remove.
The tradeoff
PDF Limit starts with a text-preserving pass. Stronger compression may rasterize pages, which can make the file smaller but can remove selectable text.