I used ForScore at first, but it isn’t flexible enough for all the things I want to do. I use Readdle’s Documents and PDF Expert—both for iPhone and iPad. (PDF Expert is also available for the Mac.)
Documents, a free app, is where everything resides.You can throw anything into it, including Microsoft Office documents, PDFs, and images such as JPGs and PNGs. You can play different kinds of audios and videos, including MP3s, WAVs, MP4s, and MOVs. And you can extract ZIP files. For a thorough explanation, check out this Tech Republic article: How to use Readdle's Documents app as a file manager for your iPhone or iPad - TechRepublic
PDF Expert is for creating and editing PDF documents. It’s free to download, but after a 7-day trial period it’s a $49.99 annual subscription. It’s pricey but well worth it, especially if you work with PDFs on a regular basis. I find working on the Mac easier for creating documents. However, you can do everything using just the iPad.
The apps work seamlessly together. When you open the Documents app, all the features of PDF Expert are ready to use on whatever you have stored there. I use it to mark the sections of a transcript, highlight passages in books, and bookmark parts of texts. It’s great for adding chord alternatives and notes/comments on a lead sheet or transcription. And it works with the Apple Pencil, if that’s the way you work.
It’s bookmarking feature is powerful. It not only bookmarks, but it shows the table of contents of books in PDF format for quick navigation, and you can also see all your annotated (highlighted) passages. The more you work with it, the more ways you’ll find to enhance your workflow.
Hope this gives a few ideas that might be helpful. You should at least give Documents a try.