When I press PgDown and then PgUp the reading position does not return exactly to the position where it was. (Even despite I am far away from the end of the document what would interact with PgDown move.)
This is counter-intuitive and inconvenient.
I used to use to press PgUp and PgDown several times to move to the remembered place in the document (moving between two different reading points of the same document). It does not work well as expected.
Additionally, you should add quick bookmarks in the document for the same purpose.