Adding this because I didn't know about it for awhile and maybe someone reading doesn't know - iOS notes includes a pretty good document scanner with the iphone camera. Open a new note and press the camera icon, you can scan multiple pages with the camera and it turns it into a reasonable PDF.
For some introductory topics that I don't want to invest a ton of money into but want to know more, I've gotten good mileage on lynda.com, free access via my library.
Some videos are quite old so for topics that move quick like technology, be sure to watch the version of the course that is 1-2 years old
The sad thing is, all* their tier 1 mobile apps are still ios/android native and are not going to be rewritten, so they still need native devs. But after this blog post native devs will be a lot harder for them to find and hire.
* except for POS for Android which apparently has been rewritten, but not yet released, in RN.