Double-tap any text to start reading from there. Better voices: iPad Settings → Accessibility → Spoken Content → Voices (download an Enhanced or Premium voice).
Apple Books keeps its PDFs in iCloud Drive, in a folder called Books. Tap Add books…, go to iCloud Drive → Books, tap Select, choose the PDFs (or Select All), then Open. If your iPad lets you pick a whole folder, Add a folder… takes the entire Books folder in one go.
Each book is copied into LECTOR on this iPad, so it opens instantly afterwards, but it also uses storage: the iPad downloads anything that is only in iCloud first. Add it to the Home Screen and use it from there, because Safari may clear a website’s storage if it goes unused for a few weeks. Books already in the library are skipped, so adding the same folder again only brings in what is new.
Open PDF… and Add books… also reach Dropbox (turn it on under Files → ⋯ → Edit).