Your scanned PDF a nightmare to read? FixMyScan takes a whole scanned book and gives it back the way it should have come: white pages, sharp black text, margins cropped, every page searchable, and a file small enough to email to your Kindle. Upload once and the whole book is done together, not page by page. A 40 MB scan can often drop to about 6 MB in minutes. It reads well on Kindle, Kobo, Kindle Scribe and reMarkable, and DjVu files are converted to PDF on the way. It's $9.99 a week or $79.99 a year. Files are deleted 24 hours after your book is ready.
I kept downloading scanned books from archives that were grey, huge and unreadable on my e-reader, and fixing one meant chaining three desktop tools. FixMyScan does the whole book in one upload in the browser.