Since I don't know if you can add tags to various text formats like you can to audio formats, for portability I suggest saving a file in the document's folder and/or, less portably, in a central file. Or it could simply append to the filename, like I do, a string of the text next to the cursor or at the top of the page, and find it when reopening it, which would make it work like a tag, and perhaps the date last opened and perhaps total time spent reading. And it could also restore the window position and size and send commands to remove toolbars etc. if the app doesn't remember them. An option to choose which method(s) to use.
You would teach the app by mouse where the text window is (or the page number field if needed), and tell it what key commands are needed to operate Find (or Go to Page) (presets for common software). It reads some of that text (or page number), stores it and when the file is opened, it issues a find/go to page and you're back where you left.
Also if you have multiple files open (Foxit Reader, Notepads, Word, Browser) it would optionally store all of their state simultaneously, as a session, so when you open one, every other file is opened, too, and their positions restored.