Add a Specific File as a Tab in Teams

You can pin a specific file as a tab—just make sure the file has already been uploaded or shared in the chat or channel where you want the tab to appear.

1.  to the channel, group chat, or one-on-one conversation where you want to add the tab.
2. Click “Add a tab [+]” and then choose the app associated with your file (e.g., Word, Excel, PDF, etc.).
3. When the app’s description screen appears, select Add.
4. Follow the on-screen prompts—each app may vary slightly. Note the option to post about this new tab.
5. Choose the file you want to display and click Save.

Once added, anyone in the chat or channel will see the tab and can view or edit the document (for Office files with permission) directly from the tab.

Source:
Use an app in a tab in a channel or chat in Microsoft Teams