Your title mentioned a problem with "downloading" files while the text indicates "Open a pdf file online".. Those are two different things.. So I'll ask: "Have you tried downloading/saving the pdf file to your computer, then opening it?" Opening pdf files online isn't the safest thing in the world, as malware can be contacted by opening unknown links on websites, so it's always a good idea to download the files to your desktop where they are automatically scanned by your antivirus program. It also allows you pdf program to open them easier, versus opening them through a browser.
Which browser are you using when you are trying to "download" or "open" pdf files? If you want to open such files in a browser window, you'll need to make sure the add-on is enabled for the program in that browser.
And most importantly, do you have a program installed, such as Adobe Reader or Foxit, which opens pdf files? If not, then you'll need to install one.. If you do have such a program, try re-associating the file type to the program like this:
Hope this helps.
When I go to open a pdf file online to view and then save, I get an error message that says that the pdf is "not a valid Win32 application". How can I fix this? It only recently started doing this.