I don’t know if this is a bug or how remnote works so far… but the pdfs I upload to my documents are only accesible on the device I did it on. I shuffle through devices all the time and not having the pdfs come along is a pain… it might just get me stop using the premium version 
I actually found a workaround for this issue.
All the files are stored in user/remnote//files All the pdf files here are renamed to some intelligible characters.
So the workaround is that when moving between devices you can copy these files and paste them in the same path in the other device and you are then able to find all the pdfs are synced to remnote.
This is just one way that works for me. I don’t completely understand how remnote functions in the background.