Google Docs introduced the Tabs feature in Google Docs. You can create multiple Tabs inside your Document. This feature is not supported for Mail Merge and this article will explain how to workaround this issue.
When merging emails from a Google Doc body the words 'Tab 1' appears at the very top of the email. Something similar happends when merging the Google Doc to a PDF file. The 1st page of the PDF just says 'Tab 1'.
The issue occures when the default 'Tab 1' is touched. Even if edited and renamed back to Tab 1, the issue will occur. Same thing will happen if you add Tab 2 or more. Even if you do not put any content in the other tabs.
This is a bug documented in Google's IssueTraker here. Please press the link and and vot +1 and/or click the Star button. When more people acknowledge the issue Google acts faster to resolve it.
The following are steps which serve as a workaround to the isssue.
Create a blank new Google Doc,
Do not touch the default 'Tab 1' from the new Doc.
Paste the content from the original document to the new document.
Try again with a test email or generating a PDF.