So, I tried one pdf songbook with sheets (about 150 pages ~26MB). Pages have no text just fullscreen images of music notes which are converted to pdf. Reader struggles to load pages. You get "caching page xy" when you are first time on any page.
Then it only caches a few seen pages so when you turn back it's instant but if you turn more than like 3 pages backward it forgets the cache and you see "caching page xy" screen again for "few" seconds. Is that suppose to work like that?

Turning pages with "Moukey Wireless Page Turner Pedal" works fine in default app and Moon+ Reader.