Two other error messages we show many of you say:
The uploaded file does not seem to be a valid PDF file
The uploaded file seems to contain errors and cannot be unlocked
What these really mean is that the underlying software encountered something unexpected while processing your file.
Well-formed PDF files conform to the PDF specification. Malformed PDF files, for whatever reason, do not. For example, some PDF programs may deliberately generate PDF files that are malformed; they can deviate from the specification to add nifty features to the files that only their program supports. Other PDF programs have bugs and inadvertently fail to fully conform to the specification.
In addition, PDF readers vary in how they deal with PDF files that do not follow the PDF specification 100%. Some may simply ignore it, while others actually try to fix the errors for you. Either way, there is a good chance that what really is a malformed PDF file opens just fine in some PDF readers.
When you encounter these error messages, you may want to double check that the file you uploaded can be opened in a PDF reader. If not, your copy is evidently corrupt and you should, if possible, try to get a new copy.
If it opens fine, there is unfortunately nothing we can do in the short term to fix it for you. In the longer term, hopefully the PDF software industry as a whole will keep working on improving its adherence to the specification. Only this will keep PDF (Portable Document Format) files truly portable.