Feature summary (what you would like to be able to do and where):

The idea of saving positions is very cool. Unfortunately, most books I know do not have centered text, and therefore scans are not necessarily centered either. Even if you have an official PDF, in the printable version the left and right pages have different margins.

Therefore, it would be best if the pan position was stored separately for even and odd pages.

To be more precise, the whole thing can be stored collectively (initially). If the user enters page 3, it is stored collectively, but with the annotation that the setting applies to odd numbers (e.g. add "was":"odd"). The user then goes to page 4 and performs an offset correction (pan), so we store this as an even page offset and rewrite the previous offset as an odd specific offset. Similar to when user started with even page just the other way around. When you only have a common setting you can just append "was":"odd/even".

Use case(s) (list the steps that you performed to discover that problem, and describe the actual underlying problem which you want to solve. Do not describe only a solution):

Example book that require separate pan (large, different margins):
https://pl.wikisource.org/w/index.php?title=Strona:My,_dzieci_sieci_-_wok%C3%B3%C5%82_manifestu.pdf/7&action=edit
https://pl.wikisource.org/w/index.php?title=Strona:My,_dzieci_sieci_-_wok%C3%B3%C5%82_manifestu.pdf/8&action=edit

Example scans, smaller difference, but also significant:
https://pl.wikisource.org/w/index.php?title=Strona:Marcel_Proust_-_Wpsc01_-_W_stron%C4%99_Swanna_01.djvu/10&action=edit
https://pl.wikisource.org/w/index.php?title=Strona:Marcel_Proust_-_Wpsc01_-_W_stron%C4%99_Swanna_01.djvu/11&action=edit

Benefits (why should this be implemented?):

Any wikisource I think.