Maybe try closing the Sprint and PDF reader app, and then open the Sprint app again and try to view the PDF. I've noticed sometimes that an app has to be closed/opened again to "see" that another app it is expecting is available.
In this case, the Sprint app may be "querying" the installed apps your your phone to see if a PDF reader is present, and if not then the view bill function may not work. I am assuming that the Sprint app is relying upon a separate PDF reader app and does not have such capability built in.
On my S22 Ultra, Samsung Notes, OneDrive, Google Drive, Dropbox, and Office show as available apps when I select a downloaded PDF by long pressing on the PDF name, then clicking on the three dots "More" and then selecting "Open with". This approach might be an option if the Sprint app allows you to download the PDF, or the Sprint app still doesn't work after closing/reopening.