By default, macOS only allows you to open apps from the official App Store. If you have this set as your default, you will be seeing the warning when you open PDF Reader for the first time.

If you cannot open PDF Reader, please change your system preferences with the following steps:

Allow Open apps not from Mac App Store

1. Open System Preferences

2. Select "Security & Privacy" and select the "General" section

3. Click on the lock and enter your password so you can make changes

4. Change the setting for “Allow apps downloaded from” to “App Store and identified developers” from just App Store

If your Mac still stops you from opening PDF Reader, please follow these instructions:

Open PDF Reader by overriding security settings

1. Open System Preferences

2. Go to Security & Privacy and select the General tab

3. If you've been blocked from opening an app within the past hour, this page will give you the option to override this by clicking the temporary button “Open Anyway” (This button is available for about an hour after you try to open PDF Reader.)

4. You'll be asked one more time if you're sure, but clicking “Open” will run the app

This will create an exception only for PDF Reader (won't apply to other apps), so you can open it in the future without repeating this process.