You might be referring to these or similar:
iOS - forScore or Unrealbook or iGigbook
Android - MobileSheets
(PC - MobileSheets)
Designed for musicians, they display PDFs (and other formats, depending on the app) and can be used in the foreground (iReal Pro playing in the background). You can use them for notation, lyrics, exported charts from iReal Pro, etc. You add metadata to each song (like composer, key, style, band name) then sort, organise, categorise, make set lists, search ... and make annotations with stylus, finger or on recent models of iPad, use Apple pencil.
https://forscore.co
http://www.diystompboxes.com/unrealbook/
http://www.zubersoft.com/mobilesheets/
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