These rules come from “Step 6: Upload evidence” of the “How to Fill Out and File a Form Online” section of USCIS’s official “Tips for Filing Forms Online” page — that is, they are documented for evidence you upload while filling out a form online in a USCIS account. The same page describes a separate “upload a completed form PDF” flow for certain forms and states no size cap for it, so always check the filing instructions for your specific form. Within the documented flow the rules are simple but strict: files over the size cap simply will not upload.
The published guidance also asks for clear, readable scans or photos of each document, and certified English translations uploaded alongside any foreign-language documents. Those are content rules; the table above covers the mechanical ones our tools can fix.
If your PDF is over the limit, the most common cause is scan resolution: a 10-page document scanned at 600 DPI in color can easily exceed the cap. Compressing the embedded scan images — or re-scanning at 200–300 DPI — brings it under while staying readable.