Hi @Alex monteiro
According to our tests, if you open a pdf file in the Teams channel as Open in app, the file will be opened as OneDrive:
The reason you don't have the issue in other channels is that the channel is synced correctly with OneDrive.Files in private channels are saved in a separate file (Files in private channels are saved in a separate Folder (different from standard channels).
Other types of files can be opened directly with apps (Word, Excel, etc.), which is different from PDF files.
So, you need to complete the synchronization to open the PDF file.
If the answer is helpful, please click "Accept Answer" and kindly upvote it. If you have extra questions about this answer, please click "Comment".
Note: Please follow the steps in our documentation to enable e-mail notifications if you want to receive the related email notification for this thread.