Mail merge export issue
Based on the "Notice" in the comment you posted above, it sounds like the issue is one of headers already being sent.

When you generate a .pdf/.docx/.odt file, the web server sends the exact set of bits and bytes necessary to generate that file.  However, it assumes that no other data is being sent.  If, before the document is generated, additional data is sent, you'll get a corrupt document.  It looks like that's what you're experiencing.

Check out the answer here: Error when printing PDF letter or mailing label from search results list.  It gives links that explain how to suppress notices on your live site, which should resolve this issue.