Whenever Word opens a file that isn't a native Word file type, a conversion occurs. There is no way around that. In that case, do the following: Right-click on the icon for a PDF file. On the context menu, click Open With, and then click Choose Default Program. Select your PDF viewer if it is listed, otherwise click More Options, scroll to Look for another app on this PC and click it.
Or you can prevent PDF documents from being converted to word by following these methods.
1. Open the document in Acrobat.
2. Right click on the document and click on "Document properties".
3. Go to "Security" tab.
4. Select "Password security" from security method.
5. Click on "Change settings".
6. Check the box which says "Restrict editing.