How to compress a PDF without making it unreadable
Keep the original and define what must remain readable. Check whether images, scans, duplicate pages or complex graphics are making the file large. Begin with the mildest useful compression and use stronger image reduction only when a real email or portal limit still is not met.
Verify the downloaded copy
Open the result, check the smallest text, footnotes, equations, diagrams, signatures and QR codes, then confirm page count, orientation, searchable text and finished size. If quality fails, return to the original instead of repeatedly compressing the reduced copy.