[Dolansky]

Indeed, it is not the reader, some pdf are difficult coded. what you see is not what you get - with some pdf. Hence PDF are gerenerally extrem heterogen, some are smooth to work with - some are really hard to handle.

I got recently a complex difficult pdf, where the annotation and handwriting is slower than the other ones. The oponent device Pocketbook 912 need hours to calculate,if one want to mark text in it - The M92 get the Text-Layer in seconds but delayed.

To overcome those hard nuts to crack is the Adobe Acrobat Full Version as a very useful tool

Adobe Acrobat -> Tools -->(Document -processing->Scanned Document optimze {OCR-searchable,Target Language, Filter options, Quality-Settings, etc.})

The text-highlighting and handwriting works now very fast - very close to real-time-feeling.

by the way - the so called problem, with vanished spaces between word with some PDF, depends if the word-position in the Text-Layer uses x/y coordinates, therfore they dont need space's

But to achieve white spaces in the Highlighted Text->View Notes, again could help out Adobe Acrobat with the action-assistent->generate barriere free PDF, the white spaces between words will show up in your notes, as well working in the dictionary function for example.