Microsoft Word Web App fails to open a PDF

Hello all,
I was using the pretty old version of sharepoint for long time, without any issues. But now things changed, it asked to me update the version so. After updating I am getting this error message. What will I do now? Some documents are created in old version and some in new, how will I correct this problem?

Sorry, Word Web App ran into a problem opening this PDF. To view this PDF please open it in Microsoft Word.
