In order for documents to be successfully signed, they must contain a created date and be PDF Version 1.4 or higher, must not be password protected and be less than 25mb and 200 pages in size. This information can be found in the advanced document properties of the PDF file - select File>Document Properties. The screen shots below are PDF files created from various systems - the first three would be suitable for signing, but the last one is not.
Workaround Solution
If you have documents that are downloading from an online program with an early version of PDF, a solution is instead of downloading to Print the document to a PDF file instead - there are many free PDF printers available for download, one of the most popular is Cute PDF Printer.