Hi there, I just found out that the preview app on my mac shows the pictures or pdfs I looked at before. As seen in the picture below (Update: somehow I get an error when uploading this pictures, so please just try it yourself and look) it shows the last six things I’ve watched. If you want to try it yourself just open something in preview and then go to app-exposé which you open by sliding four fingers up on your macbooks trackpad (if enabled) or control + arrow down (at least at my mac). You can select and open the pictures (I just call it pictures, because mentioning everything what preview can open would be too long) which are shown down, the last six as mentioned before.
I noticed this one public beta before, so it must have been Public Beta 4, currently I’m running the 5th version.
I think it’s a nice feature and though I didn’t use it yet I like it somehow.