Use an online “compress PDF” tool
Open the PDF in Adobe Acrobat and choose: File → Save as Other → Reduced Size PDF
In Adobe Acrobat, use: Tools → Optimize PDF → Reduce File Size
If you have scanned pages, rescan at lower resolution (e.g., 150–300 DPI) and save as PDF again
Convert images inside the PDF to lower quality (reduce image resolution and/or JPEG quality)
Remove embedded fonts (subset fonts) and avoid embedding full font sets
Delete unnecessary objects and pages, then re-save the PDF
Flatten form fields and annotations before saving
Convert the PDF to a lighter format (e.g., re-export from the source app as “smallest file size”)
Use “linearized” or “web optimized” PDF settings when available
Reduce PDF version (if supported by your workflow) to improve compatibility and size
Recreate the PDF from the original source documents using export settings for smaller size
If possible, avoid OCR text layers that are duplicated or overly granular
Use a script/tool like Ghostscript to optimize and compress PDFs
Check for and remove attachments embedded in the PDF
Ensure there are no duplicated images or unnecessary transparency layers
After changes, re-save as a “new” PDF file to avoid carrying extra metadata