Why PDFs for Screens? (Technical Details): Benjamin-Newton.com

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Why I Use PDFs on my Website - Technical Details

PDFs can contain vector graphics (much smaller size on disk / drive and also at the highest resolution possible). Epubs can too but they won't work on Kindles and you have to create the document manually to use SVGs.

PDFs can be much bigger on slow devices because they only have to load one page at a time. PDFs can be made accessible and you can fairly easily change them back to the same source file (if they choose that option to tag them when they are created). PDF is an older format that is very mature and stable in its specs.

There is no real advantage of Epubs over PDFs for graphics books. Most of the ebook readers can also read PDFs. The Epub format for pictures jumbles up everything in the code. The user also can't modify these kind of Epubs like changing the font size and colors.