The PDF reader included in firmware 6.xx is total bullshit !
There is no issue with the hardware.
The solution is to downgrade to firmware U632_5.19.832, to do this use the 'Manual Update' procedure described on p. 93 of the User Manual.
You may still encounter problems with some pdf files (scanned documents), in this case you may try to 'OCR' it with
OCRmyPDF.
It is a good idea to install the latest release of
KOReader for the missing formats.
Hope it helps