Are you still getting sync errors? If not, are you syncing with the machine where you added the PDFs? That's the only place they currently are, so that's what you need to sync to make them available online and on other machines.

@RosemarieParisi: There's nothing here to "happen" — it's just a question of understanding how file storage works. If you were at your storage quota, additional files couldn't be uploaded from any computer. Once you upgrade your account, that doesn't magically make all your files available on computers where they never existed. You need to actually sync on the computer where the files were added, which would be the only place they exist, and you need to make sure file syncing is enabled. See Files Not Syncing if you need help with this.