Update: To restate my goal: I have a library of PDF files which get occasionally updated by new pages being appended. I would like to have this library available and searchable from within Evernote clients but also be available through another cloud service. As files get modified I'd like for them to automatically be synced to Evernote.
Among the suggested solutions, the Import Folder feature always creates new notes so that didn't work and the Google Drive integration is half baked right now, the pdfs I uploaded to Google Drive aren't searchable from within Evernote. They also show up as links on my PC and iOS devices so that was a no-go.
What I settled on is the following:
1. Place my PDF library in a Dropbox directory, let's say C:\Dropbox\PDFLib
2. Copy the entire library into an Evernote Import Folder and wait for the sync. I deselected "Keep" in import options so now the import folder is empty.
3. To add more content to some pdf in the library I find it in Evernote, right click Open With and select my PDF editor. Typically I append another PDF. When I'm done I click Save once to save the Evernote copy followed by Save As into C:\Dropbox\PDFLib and overwrite the existing file.
It's a bit clunky but it allows me to have a versioned copy of the PDFs in addition to having a copy on Evernote. Sounds like I could use Cloud HQ to do this but because Cloud HQ syncs AFTER Evernote has had its way with my files I feel this is the safer approach.
Ideally Evernote would provide a simpler way to do this, for example by allowing an "overwrite" option in the Import Folder. This way, I could just make an Import Folder out of my C:\Dropbox\PDFLib directory and be done.
Thanks to everyone who took the time to reply and make suggestions.