How can I delete an old file in Acrobat?

Hi @Deanna88 

Hope you are doing well. 

Sorry for the delayed response. As per the description shared that you can't find an option to delete files in Acrobat Reader mobile. To delete files:
1. From the list of files, tap  to the right of a file.
2. Tap 

3. Tap OK to confirm the action.

Also, you can refer to this help document for more details: https://www.adobe.com/devnet-docs/acrobat/android/en/managefilesfolders.html#delete-files

Let us know if you face any difficulty.

Thanks,

Harsh