Cache pruning on Android is a system feature and you don't need to think about it. But there are situations when it's useful, for example:
- You want to regain used space
- You have performance issues
- PDF Viewer application misbehaves (crashing, ...)
How to do it?
- Launch Settings
- Go to the Storage section
- Select Internal shared storage - Other apps
- Search for the PDF Viewer application
- Tap on the Clear cache button