In order to change the default reader for PDF files (to not open PDF files with Firefox's internal PDF reader), follow these steps:
- Click the menu button and click Options (Preferences for Linux and Mac).
- In the Options (Preferences for Linux and Mac) tab (window for previous than 38.0 firefox version), select the Applications panel.
- In the Search field, type PDF. You should find Portable Document Format (PDF).
- On the right hand side 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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