Tagging your PDF document has the same effect as putting it in a folder, but without having to define new folders.
Click on Docs in the main navigation. To the right of each document in your library, you will see three dots (...). Click on those to automagically bring up an input box where you can add a tag.
Once you have added your first tag, refresh the docs listings. Your tag will now appear the top of the listings. You can search for it in the search box, or simply click the tag. The tag acts just like a folder, tag any other file with the same word and you can list all your documents by that tag.