Set Default Apps in Windows 10
When you receive an email with a .pdf attachment, you may encounter an error opening that attachment as it will be marked as "unsafe". This is caused by Windows 10 attempting to open a file from the Internet within its Edge browser. The easiest way to fix this would be to set Adobe Reader as your default program to open this file type.
Before you begin, please visit https://get.adobe.com/reader/ to download and install the Adobe Reader application. If you have not yet done this, you will not be able to complete the steps below.
1) In the search bar, type Settings, then click on the top option.
2) In the search bar, type Default, then click on "Choose default app for each type of file". It may take a minute to populate the list.
3) Scroll down until you see the .pdf file type on the left.
4) Click on the current app (should be Microsoft Edge).
5) Select "Adobe Acrobat Reader DC" from the list.
6) If you are presented with a "Before you switch" box, go ahead and click the "Switch anyway" link.
7) You can now close the "Settings" window.
PDF files should now open properly without the warning message.