TOPIC:

when a document is open in the default mode (vertical), it doesn't fill the screen vertically, some of the second page is viewed under the first one, the PDFView considers himself viewing the second page not the first page, so the onPageChanged will be fired once the user goes to the third page and it won't be fired when the user scrolls to the second page because it consider itself that its showing it already.
this makes a problem when you implement a page counter in your application.
P.S : please note that you have this problem in your application : play.google.com/store/apps/details?id=radaee.pdf&hl=en
many thanks

we will check this problem for next week.

any updates regarding this problem ?

may you test call-back "OnPageDisplayed"?
this call-back should invoked every time, when page displayed.

I added a Toast message to this method to show the current page, it keeps showing the toast message, the function onPageDisplayed is being called every second, even if I don't change the page.

whats the problem in that ?

yes, it call-back all times for page moving.
to get current page for single page view:
you may check if right side of page is in left half of screen.
and record pageno call-back to you.