[Zubersoft]

Image files often load faster than PDFs (especially if the PDFs are not optimized). They take up more space usually, depending upon the compression algorithm used in the PDF to compress the images. You only have to deal with one file in MobileSheetsPro instead of multiple, and you have more control over page ordering. With PDFs, you can repeat pages by just modifying the page order, or cut out pages if needed. With image files, you have to add the same file multiple times to create a repeat.

I would recommend PDFs over image files unless the loading times become unreasonable. Even then, I would usually recommend just optimizing the PDF to reduce the loading time over having to convert it to image files (especially if the PDF has a lot of pages in it).

Mike