I have quite a few papers and textbooks that I store on my Mac. The scanned ebooks are usually very large and when I open them with my favourite PDF reader they are quite slow to browse through.

I’ve found that if I PDF compress these scans they take up way less space (from 100s to few MBs) and Preview is much snappier when browsing/searching through them.

The compressor for PDF files online: https://www.sejda.com/compress-pdf