I am currently working on a graphic novel project that I am planning on submitting to apple books as an epub file. The files were created in adobe illustrator and subsequently, linked into adobe indesign to create the final published product. I have noticed that the image quality when exported to epub is far inferior to when that same file is exported to a pdf file. That is regardless of the alterations that you make to PPI or other options (such as altering between PNG, JPG and other file formats) prior to exporting to epub. I have adjusted the PPI to 300 and still the color of the document and the overall image quality lack. I find myself having to ***** open the epub to exchange out the indesign generated files for files that I optimized in illustrator (which can be a painfully tedious process if you have a ton of pages/images to optimize). These newly optimized files look far better at a fraction of the size of the indesign generated files. I located this forum thread from 2011 that actually seems to correspond to my problem (https://forums.adobe.com/thread/871355).

Please let me know if you need any additional information.

Regards,

Tom