how to persist paging info

If it's a task within a running process instance (and you're looping back to it in the process flow), you can simply pass the current paging info value out into a process variable and pass it back into the task form.

If, however, you're referring to somewhere not process-backed (like a record or site interface), it gets a lot harder.  Pretty much the only way I've found in the past, (which is difficult enough that I only recommend anyone implement it if it's absolutely required and you fully understand the appin expression coding required,) is to implement a special database table that stores user preferences (on a user-by-user basis), which gets automatically queried when the form loads to see if that user has any "default" values, and which gets written to whenever the user interacts with the paging of the grid in question.