The pdf reader sidebar (in your remembered files) is open by default. If you want to change this behaviour do the following:
Open "about:config" in a new Firefox window. A warning message will appear. Just accept the risk and continue. A search bar will appear now. Search for the following option: pdfjs.sidebarViewOnLoad. After you clicked "enter", you should see the option you searched for. On the right, you will see an edit icon (pen). Click it. Now you are able to change the value from -1 (open) to 0 (closed). Save the changes by clicking on the blue checkmark icon on the right. Now you changed your pdf reader default behaviour from open to closed.
Note: Chrome doesn't allow to change this behaviour for now, this guide is only valid for Firefox.