As you have mentioned that you are not able to locate the files on your device, please confirm if PDFs appear under the "On this device", "Document Cloud" or Other accounts added under Files in the Adobe Acrobat Reader applicaiton.
Adobe Acrobat Reader mobile application only helps you to view the PDFs in the application. The PDFs do not get saved in the application on the Android device. If the files appear under "On this device", they might be saved in the Download folder or on File Manager on your device.
If you are still not able to locate the files, you can use the share option in Adobe Acrobat Reader mobile application to save them on an alternate location.
And if they appear under the Document Cloud storage on the device, you can access those files online from here Adobe Document Cloud. You can open the link in any browser on your desktop and download files from there for backup.
If you are opening files from any third party account added under Files in Acrobat Reader application, then you can access those files either from their application (if the app installed on your device) or from their cloud storage.
Let us know if you need any help or have any questions.