Here's a feature that will come in handy when Google Docs adds support for syncing: version history for files. Until now, Google offered this feature for the documents that could be edited in Google Docs, but not for PDFs and other files.
"Previously, each time you updated a file, you would have to upload the new version as a new file with a new URL, re-share it, and put in the correct folders again. When you select the Add or manage versions option on any file, you can upload new versions of a file, download previous versions, and delete older versions."
Unfortunately, this only works when you manually add a new version of a file. If you upload a file that has the same name as an existing file, Google Docs doesn't add a new version and you'll end up with two files. Dropbox is smarter and it automatically updates the existing file. Another issue is that you can't preview the versions of a PDF file in Google Docs, since the only option is to download the files.
7 hours ago