In general when this program (at least the iOS version) works at all it usually works well, but it often doesn't work. The interface -- color scheme, default font/margins/size, &c -- is easily customizable. It has an easily usable TTS function, itself customizable in speed, tone, & accent, & generally effective, though it occasionally interprets an English word as in another language if you don't set it to only use 1 language (eg it assumed that the word "war", in a section of pure English, was German), & even after that is done it sometimes misinterprets words (eg "just say no." was repeatedly read as "just say number", & "est" in "_ delenda est" as "estate") & it has a curiously inflexible intonation of certain phrases (eg "at all" is always pronounced very emphatically, & "war" is often read as "waah" even when set to an American accent).
Regarding the specific file types I used it with:
PDF: Displays pdf files 1 page at a time & works well aside from the fact that it will only turn a page if the current page is fully zoomed out, which often makes the text too small to read. The TTS is flawless with text-formatted pdfs & usually correct even with those made up of scanned images of text.
RTF: Displays a large-sized page so that the right side is cut off of a phone's screen. Totally unusable for text reading, though the TTS still works fine.
HTML: Displays well, with a customizable display as described above. Larger files take a long time to load: one multi-megabyte ebook took more than a minute to load each time I opened it. The TTS often makes mistakes: it reads some punctuation, especially the em dash & directional quote marks, as "to the power of"; ASCII &charname; codes for non-ASCII characters (eg é for é or — for —) are often read as the letters making up the code or also turned into "to the power of".
EPUB: When it works, it usually works as well as for html minus the long loading time. However, with at least half of the epubs I try to open (which are perfectly legible in Marvin or Firefox's Epubreader extension), Pocketbook pretends to load them for a long time before showing an error message along the lines of "errors in file opening" & thereafter stubbornly refuses to display any text. (This is consistently the case with autogenerated epubs from several large sites including Archive.org & AO3; the necessity of finding alternate copies or using html eventually annoyed me into changing to a different program.) The TTS usually works without the errors described of html files, though in some books it occasionally (once every 2-3 hrs) crashes & the book has to be closed (provoking another vague error message, this one without immediate practical effect) & reopened before the TTS will start working again.
So, essentially, this app is flawless for TTS reading of pdfs or of those epubs formatted according to its inscrutably exacting standards; but if you want to read displayed pdfs or html or rtf files, or if you want epub reading to work consistently, you should look elsewhere.