Every Mac app remembers open documents, tabs and windows. The one exception is currently Papers. It would be really great if this could be changed, please.
The reason in my case is that I frequently have quite a lot of papers open, and when I need to close Papers for whatever reason, there is always the risk that they'll be closed if I forget to tick the little "remember these" checkbox, in which case I might lose track of an important article.
You can have the Papers desktop remember to restore your tabs by heading to Papers > Papers Settings > General > Restore Tabs