Set List Maker currently supports documents in PDF, Word, PowerPoint, Pages, Keynote, RTF, plain text and JPEG format. From what I've seen, PDF and Word are the most commonly used formats. Some other songbook apps only support PDFs.
I always recommend the PDF format because it renders more accurately than Word. PDF is also required for saving document zoom positions, and supports custom background colors. In general, iOS supports PDF better than the other formats. Most recently, iOS 7 introduced problems with Word files that caused crashes for some users and took many hours of troubleshooting to resolve.
So the question is, what would you think if I dropped support for all document formats but PDF (and possibly JPG)?