Suppose the newsletter appears as a link, you can try right-clicking on it. From the list, choose "Save link as" and see if you can save the PDF file on your local hard drive. If it doesn't work, you need to deactivate the Adobe add-in in your classic Firefox:
1. Choose Tools > Add-ons.
2. In the Add-ons Manager window, click the Plugins tab, then select Adobe Acrobat or Adobe Reader.
3. Choose Never Activate.
Now, when you click on the same PDF newsletter link, Firefox should open a window and ask you whether to save the file of open it with your default PDF viewer.
Please note that if you upgrae your Firefox to Quantum, the process will be completely different.