We have been seeing PDF files repeatedly exporting incorrectly from google’s servers, which causes an “empty docs” error and which eventually causes a CAPTCHA error. This has been happening lately to a lot of applications that use the documents API from Google and is a bug in the Google Docs servers. This has been happening since about last Wednesday; Google tells us that they are working on it.

Update: Looks like this is working now. See Here for Google’s progress in case the issue comes up again.