Yes you can (if I understood your question correctly). Press p and then navigate to the target file and then press p again to create a portal from the source PDF to the target PDF. (you can press tab to automatically jump to the target document if you need to)
> Can we export the marks/bookmarks/document state to a plaintext-compatible file?
Currently not but I could add it as it seems that a lot of people are requesting this feature.
> are marks single-character only? And can you not jump straight to a bookmark, only bringing up and searching the bookmarks context menu?
Yes. You are correct. Marks are meant for quick jumps around documents and Bookmarks are meant to be for more persistent situations (for example you can bookmarks an an exercise in a PDF so you can return to it at a later date when you have more time)
> "Set bit 13, CRCEN, before turning on bit 6, SPE, see ref manual page 808" or in your schematic that your PCB needs to "Route for additional 4.7 uF point of load capacitance within 25mm/nH/mOhm, see datasheet page 69".
I think portals are perfect for this use case because, as I said, you can create portals across different documents.