Here is how I fixed mine. I opened the document, which it open in word. I saved the document as pdf format. I then opened the document from place I saved it. It opened as pdf, but with garabe. I then right click and properties tab. File associated with to open changed to pdf click apply. When I went back to my email message the file was in proper format. I then opened it. It was fine.
Even you have installed PDF reader, you may also cannot open the pdf files. That is possiblely your PDF reader is the older version, you need to download the lastest version of pdf reader