How to fix this: If you are using Firefox on Windows, your browser needs to be set up to display PDFs using Adobe Acrobat outside the browser. To check, go to Tools->Options->Applications and check what the action is for "Content Type: Adobe Acrobat Document" (all PDFs can be considered Adobe Acrobat documents). It should say something like "Use Adobe Reader". If it says "Use Adobe Acrobat (in Firefox)", change it to "Use Adobe Reader". Then click OK and re-click on the PDF button in Cytobank.
If that doesn't work, make sure Adobe Reader is installed on your computer.
If you are using Firefox on a Mac, the PDF should be automatically opened in Preview.
