My musical life is totally contained within forScore on my iPad Pro. It is an absolutely awesome app and the primary reason why I don’t try Android tablets.
(prepare to be bored with tedious details)
All of my music is in the form of PDF documents. Many are from old scanned sheets, others from downloaded music, and still others from books.
forScore provides a dozen ways to get music from cloud services into the app, so you should be covered–use iCloud or DropBox or whatever you like.
When I buy music from online services, I use “print to PDF” to convert to PDF on my Mac and then use iCloud to get at it from forScore.
When I have a dead-tree songbook, I use a different tool: an app called Scanner Pro by Readdle. It’s a neat app that uses your iPad’s camera to “scan” documents. It includes nice de-skew features, just make sure to choose a fixed paper size such as A4 or Letter instead of letting it do automatic.
I often put a piece of Plexiglas on top of the page I want to photograph and get the lights just right and snap the shot, though it works without that prep in a pinch. The app sets the contrast perfectly so it looks like a photocopy. Multiple pages can be combined into one PDF doc. I then name the doc something like “Some song - Piano - Bb.pdf”
Then I save it in iCloud from Scanner Pro and go on to forScore to load the file.
Note that forScore has built-in scanning tools, but I am used to Scanner Pro, so that’s how I do it. Try out the built-in one first.
If you have bought a complete book in PDF, learn about the Bookmarks feature–it is a way that you can use to create new “song” entries in your library that are backed by references to ranges of pages in a PDF book–it works seamlessly. That way you can put a song in set list without having to mess around with flipping through a 90 page PDF to get to it.
Using an iPad for music is not without its faults: one particular problem is that you double the number of page turns. My wife uses a giant iPad at the piano to solve this–when she turns it in Landscape mode the app shows two pages side by side, just like a book.
Stuff you didn’t ask about…
Using Apple Pencil: Besides managing thousands of pages of music, I love that I can mark up everything. I use my Apple Pencil to write all of my bass lines directly on the sheets. There are even little annotations like “Use Drop D” or “Tacet” all over the place.
Using Set lists: If you are a working musician, get very comfortable with the Set lists feature. I play in church each week and rely heavily on it to know what has been played when and to plan future sets.
Page Turners: Look into a Bluetooth page-turner pedal. I use a PageFlip FireFly. It’s totally awesome for me to have both hands on the bass while tapping my toe to go through the song sheets.
Sharing: If you have other musicians who use the same thing, you can use AirDrop to share music from forScore, and if done correctly it will carry along all annotations and keywords.