Hi mieksje,
If you're not using a microsd card for you files/books, I'd revert back to firmware 15.2. That version would be better for what you want.
If you do use a microsd, I advise you not to revert to 15.2 as it corrupts the card in my 302 up to the point that it cannot be used anymore in any device. The 15.4 fixes this, but has unfortunately introduced some new bugs

.
There is some talk for a new firmware just recently, but that may be a couple of months in the future (my estimate, not Pocketbooks - it may take longer than that.).