When you try to open the PDF, you may see an error message like this
If this message is not eventually replaced by the proper contents of the document, your PDF viewer may not be able to display this type of document.
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This means that although you have Adobe Reader installed, you are not opening the document with Adobe Reader. This usually happens when you just double click on the file. Your default PDF viewer may be "Preview" on a Mac, or the Chrome Browser on a PC. The file will not open if you try to open it from within the Chrome browser or with any other PDF viewer.
You should firstly save the file to your computer. Then, locate the file and use an "Open With...." command. Select Adobe Reader from the list of options. If you are unsure of how to do this, just open Adobe Reader and use the File>Open command. Then navigate to the file that you have saved to your computer.