Hope you are doing well. Sorry for your experience with Acrobat Reader Mobile.
Have you tried clicking the three dots to the right of the file name to stop the conversion? This could happen due to a lag in the network connectivity to connect to the server.
Acrobat Reader for mobile uses the cloud services to provide the functionalities and fluctuation could lead to such issues.
Also, is this with a specific file or with all the files? I suggest updating the application to the latest version for a better experience.