If you uninstall Adobe Reader on your mobile device, it will erase all data stored with the app. It's just the way mobile data is stored in iOS (and I presume in Android devices as well).
You will have to get the PDF files from your backup files, I'm afraid.
Would you provide more information about the problem so that we can help you?
Do you have a desktop/notebook computer or a mobile device (iPad, iPhone, Android tablet or phone)? What is the version of the operating system?
When you uninstall Adobe Reader for iOS, iOS will erase all of the app data (including locally stored PDF documents) associated with Adobe Reader. This is true for any iOS apps, not just Adobe Reader for iOS.
On the other hand, Android will NOT erase any PDF documents that are stored in internal storage or an SD card, when you uninstall Adobe Reader for Android.
If the app that you uninstalled was not Adobe Reader for iOS, your PDF documents still remain intact on your computer/device.
Regardless of whether you have a desktop/notebook computer or mobile device (iPad, iPhone, Android tablet or phone), I'd highly recommend making backup copies of your PDF documents or other types of files on a regular basis so you will not lose them permanently.