A family of Microsoft word processing software products for creating web, email, and print documents.
Hello Gavin,
Welcome to Microsoft Community and thank you for posting your query.
Few more details would help us to provide a better response.
Does this issue occurs while inserting PDF files in all documents or is it specific to particular document?
I suggest you to start the computer in clean boot mode and then try to open Word, insert PDF file and check if it works as expected.
Starting the computer in clean boot mode would disable all the non-Microsoft applications and the antivirus which could be one of the reason for the issue.
Refer the article below to start the computer in clean boot.
http://support.microsoft.com/kb/929135
**Disclaimer:**Ensure to restart the computer in normal mode after completing the troubleshooting steps.
Hope this information helps. Post back with updated issue details for further help.
Thank you.