In response to Mad's Google search "In the context of Polish walkthroughs and guides for Goodbye Deponia (and many other point-and-click adventure games), PPM stands for Prawy Przycisk Myszy, which translates to Right Mouse Button."
Diving deeper, I found a Polish walkthrough that lists PPM twice. One of them includes this instruction: "Kliknij na pierwszego Rufusa po prawej, on ma kwas nukleinowy, kliknij na kwas PPM wekwipunku" translates to "Click on the first Rufus on the right—he has the nucleic acid—then right-click on the acid in your inventory."
Some players found this as a bug and posted on Steam, with the Developer's response here:
https://steamcommunity.com/app/292910/discussions/0/4758703546879827269/I hope this helps.