Assistant API v2 Not Reading the File Correctly (PDF)

We have Teams bot using the assistant api v2 with .NET SDK. We having issues since we introduced the file upload feature to the users. When users upload files and ask to summarise or do something, it sometimes get it right and sometimes it struggle to identify the file and talk gibberish. We also have the pre-uploaded KB with PDF and other documents, this is happening for the user uploaded files which we attached to the thread on runtime.

Much appreciate any help.