Hello!
The design intent for the Common Documents location is to be a place you can put plans that you want to link to from more than one Project. Note that typically plans and layouts can only be linked together if they exist in the same Project.
We currently do not have the ability to link to PDF's that are stored in the project browser, instead requiring that the PDF be moved to the asset browser for usage (right click the PDF in the project browser and select "Move to Assets"). We are aware this is a bit of a pain point and are looking at ways to improve it moving forward. For now we encourage putting any non plan/layout files that do not need to be linked to, but are useful to have together with the plans/layouts, into the Project.
As for the folders on the left, clicking one of them will filter your project browser to only show the Projects that are associated with that folder. The "Common Documents" and "Plan and Layout Templates" are not currently able to be associated with those folders. Let us know if that is something you think would be useful to your workflow! Note that some users prefer to not see the "Common Documents" or "Plan and Layout Templates" at all, which can be controlled in the preferences dialog under "Project Browser". See the checkbox: "Show Default Categories".
