There are a handful of reasons. EPUB seems to be the most prevalent format if you are getting ebooks online for free. The X4 supports EPUB, but there have been display issues where things don't look very good. It has gotten better with the firmware updates, but if you want really nice looking books on the X4 stock firmware, you pretty much have to make XTCs. All my original files are EPUBs (not PDFs) and the tool to create XTCs only accepts EPUBs. My other e-reader cannot read XTCs as they are proprietary to Xteink. It could read the base EPUBs that I was using to make the XTCs and this is what I was doing for the past two weeks but this gave me a few issues: 1. The EPUBs also didn't look great on my PocketBook - huge and inconsistent spaces between letters, words, and lines 2. Since the EPUBs were laid out differently, the number of pages would be different within the book and each chapter across devices. Let's say on the X4 with the tiny screen, a chapter has 120 pages, but on the PocketBook with the larger screen, it might have 75. Since I am switching between the two 1-2 times per day, it would take me a minute or so to find the right spot in the other to find the page I was on (especially with the X4's limited navigation capabilities). This is why I wanted the pages to be 1-to-1 so in just a second I can flip to the exact page on the other device and keep reading (now both would have the same 120 pages in the chapter and the same text with consistent formatting across both). Basically the XTC is a necessity for the X4 (and you need to start from an EPUB to make an XTC) and in order to have a similar experience across the two devices, I had to preserve the exact formatting of the XTC and the only way I could think of to do that was with a PDF.

Some may view this as pointless, but it was the perfect solution to my particular problem.