Windmees,
Before you shut your iPad off or leave it idle for an extended period of time, I'd recommend closing the currently viewed PDF document.
- Tap anywhere in the currently viewed PDF document to display the top toolbar.
- Tap the back arrow ('<') on the left side of the top toolbar.
When you close the document, Reader will automatically save the page location (and any changes that you've made by adding comments or filling out a form). Then it will remember the last page next time you open the same document.
You can try it several times to see if it works for you.