Click on the three dots at the upper-right corner.
If the ".PDF" associated program is not Adobe Reader, click on the current program and change it to use Adobe Reader, like "Adobe Acrobat Reader DC".
Scroll down on the right panel and click on the link "Choose default apps by file type".
Click on Apps and then click on "Default apps" on the left panel.
Click on Windows Start button and select the settings icon.
Set Adobe Reader as your default program to open PDF files: To display images (PDF) in Adobe Reader instead of displaying them within a web browser, you need to check the following program/browser settings: