As I said before, Gmail has two different versions when you use a browser.
To change between these, click the settings icon (gear in the upper right area of the screen) and use the top choice.
Since you specifically mentioned seeing the PDF viewer display, here is what Gmail looks like when you use these two versions of Gmail and view the message list when the message contains a PDF attachment:
As you can see, when using the new Gmail version the PDF attachment is shown on a separate line after the main message.
- If you click on the message (the line containing the sender name and subject), the message body is opened. This is similar to the classic Gmail display.
- If you click on the PDF attachment (red PDF icon), you will open the PDF file itself (not the message body) using a PDF viewer. I think this is what you are describing.
Is this your issue? If so, you can return to the classic Gmail version if you wish (as describe above).
Bill