I can reproduce the issue in Nightly, I download a corrupted file. This issue is also mentioned [here](https://www.reddit.com/r/firefox/comments/1c6vzew/problems_with_pdf_files_after_12501_update/) on Reddit.
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I can reproduce the issue in Nightly, it downloaded a corrupt file. This issue is also mentioned [here](https://www.reddit.com/r/firefox/comments/1c6vzew/problems_with_pdf_files_after_12501_update/) on Reddit.