Yes, all of this is possible.
In terms of side by side viewing, note that both apps need to support Split View. Among PDF apps, at least Adobe Reader and GoodReader do, and no doubt several others. ePub reading apps with Split Screen support include iBooks, Marvin, Overdrive, Google Play Books ( sort of, you have to upload to your Google books Library first). Kobo, nook, Kindle apps do not support it.
There are apps that let you create and edit ePub files, ocr images or scanned PDFs.
And iOS 11, soon to be available, will greatly improve ability to work with files and enhance Split view functionality.
I would consider the 12.9" iPad Pro - it would give you more room to work with.
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