Substituted PDF's when emailing a PDF file.

I downloaded a paid for file which is in PDF format. When I attached that file to a new email to another person, instead of it being sent it was replaced or substituted by at least a dozen PDF files currently on my HD. After this occurred I tried sending a different PDF file to myself and the same thing happened. Is this an anomaly with the Acrobat program or could it be caused by a latent malicious software that may have been downloaded with the paid for file?