Check your web-browser's settings for PDF files.
In Chrome:
- Click the '...' near the upper right
- Settings
- Privacy & security
- Site Settings
- Additional content settings (scroll down)
- PDF documents
- Ensure it's set to Open PDFs in Chrome
In Firefox:
- Click the three-stacked-lines button near the top right
- Settings
- Search for PDF
- Ensure the action for PDF is Open in Firefox