Hello Rosita Dozier,
Are you using an Android phone or an iPhone? That matters here because the way the phone chooses a default app for downloaded files is different between the two.
Microsoft has recently changed how files are handled in the Copilot mobile experience, including redirecting some files to their dedicated Word, Excel, PowerPoint, or OneDrive apps.
If you let me know whether this is Android or iPhone, and what type of attachment you are opening, such as PDF, Word, or Excel, I can give you the specific steps to try to restore the previous app-opening behavior.