Sometimes even when setting Adobe Acrobat DC as the Default, downloaded PDFs will open in Chrome instead. This is because Chrome is set to use it's integrated PDF viewer when files are downloaded by default. You will need to turn this off to make it go away. To turn this feature off, follow the steps below.
1 - With Chrome open, click the three dots on the top right corner of the screen. A dropdown will appear with options. Select Settings.
2 - A new tab should show up. In the search bar that appears, type PDF. This will give you a list of settings that mention PDF. Click on Site Settings.
3 - On the next screen, scroll down until you see PDF Documents. If you searched for it in the search bar, the word "PDF" will be highlighted. Click on this.
4 - There should be one option that says Download PDF files instead of automatically opening them in Chrome. Click on the switch to turn this on. You will know it's on if it looks like the image below.
After doing all of this, try downloading a PDF file from Chrome again. The issue should be resolved. If you are still having trouble, close Chrome and open it again and this should resolve the issue.