I have been using Voice Dream Reader for a few years now. The latest update made it the best text-to-speech program for iOS by far. It works well, has lots of voices to chose from, and reads lots of different file types. Some PC based programs, like Read&Write Gold, do a lot of this, but cost a lot more and have some other limitations.
I like to use Voice Dream Reader to read/study PDF files. I like the PDF format because it keeps the visual format of the original document (not just text). I scan and do OCR myself for many PDFs. I use the highlight and note feature within Voice Dream. On my iPad Pro 12.9", I use split screen with Voice Dream on the left and GoodNotes on the right and use the Apple Pencil to take hand written notes for documents Im "studying."
One limitation is that the in context highlighting and notes dont decode part of the PDF file. So if I want to view the PDF with another PDF reader, like Acrobat, there will be no highlighting nor notes. Thats my main/only complaint.
PNW guy about Voice Dream - Text to Voice, v4.1.0