You lose the convenience of one file though. In this case you might as well use a purpose built encryption tool rather than force KeePass into this usecase. A VeraCrypt container or encrypted overlay filesystem are a significant performance and UX upgrade since you are already willing to concede managing one file.
That's true regarding the one file convenience but from another angle it's a separation of concerns, especially considering it's a pdf accessed very occasionally.
I maybe half agree with you about the encrypted overlay filesystem but only in respect to files, not passwords though. I tend not to keep files in KP, if I need a singular encrypted file I'd probably 7zip it (7z format) with a password and encrypt the filenames. The password goes in KP as does the location.
It's also possible to create dedicate entries for each of the other KeePass vaults and set the URL field and password to the respective paths (i.e., "kdbx://PATH/TO/OTHER/DATABASE.kdbx") and passwords, then you can simply double click on the URL field to automatically open and unlock the other vaults.
The URL field in KeePass has lots of convenient features [0], but unfortunately they're quite "obscure" and not very discoverable.
[0]: https://keepass.info/help/base/autourl.html