A one-size fit all app that is priced right from iComics is iComics. I like it, even with recent growing pains. The author has been making updates so I might stick with him. It opens many, many formats and things I used to have in the Palm III days.
For just PDFs, you can’t go wrong with iBooks especially if you have iCloud. It can serve as your off site backup as well as your reader. Open a PDF in safari and click open in iBooks. Standard iPhone stuff.
iComics is my pick again.
Comic Flow is another option that is free to download but eventually you have to pay. It also hasn’t been updated since 2016. There may be a situation they address that iComics fails at. Let me know.