Ok,
I open the PDF file: 5sec.
I click on next button: 2sec.
I click on next button again: 2sec.
I click on next button again: 1sec.
** I am now on page number 4 **
Click Back button to exit from book: 2 sec.
I shut down the JBC using power interrupter: 11 sec.
Now I power on the unit: 35 sec.
I go in My Library-> ... -> file PDF
I open the file again: 5 sec.
*** The unit show me the page number 4 ***
Click on "<" and I get the page number 3: 2sec
Click on ">" and I get the page number 4: 2sec
I'm frustrated I can't reproduce the problem
Andyh2000, ProDigit, and others, could you try, please?
Edit: Valentinka I have a suspect: maybe you have the old Firmware?
Edit2: to check firmware version you can go: Settings-> Device Info: read under "Firmware Version"