In order to change the default reader for PDF files (to not open PDF files with Firefox's internal PDF reader), follow these steps:
- Go to Tools > Options (or Firefox > Options).
- In the Options window, select the Applications tab.
- In the Search field, type PDF. You should find Portable Document Format (PDF).
- On the right handside you should find an Action column. Use that to select your favorite PDF reader. In order to view PDF files in Firefox, choose Preview in Firefox.
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