[MarjaE]

Apple used to delete Roman text layers at high compression and non-Roman ones at any compression, or if users added notes or bookmarks, or if users slipped with the mouse while reading them in Preview. That affected readability on *any* device, including the Mac.

The Kindle can't open "password encrypted" pdfs. PdfGenius has a security option to remove passwords.

The Kindle can't display some images from some other pdfs. That's especially annoying when it just displays blank pages, with a note at the bottom, instead of the original book's text. Pdf Toolkit+ and similar software can fix images by reformating the pages, which they do when compressing the pdf, but they refuse to do if it would expand the pdf. One trick to reformat the pages is to take a very large single-page file, which would compress from 50 mb to 500 kb, merge it with the other pdf, compress the combination, and then split the resulting pdf.