Start with the type of PDF
Scanned PDFs are mostly images, so reducing image resolution or JPEG quality can create large savings. PDFs exported from Word or Google Docs often contain compact text and vector elements already, which means there may be less to remove.
Choose compression for the destination
Email attachments and upload portals usually have a hard size limit. Work toward that limit while keeping enough resolution for the document to be read comfortably. For contracts, forms and resumes, check small text after compression.
Keep the original
Compression is a transformation. Save the original file separately, then open the compressed copy and inspect every page before sending it.