To understand the root cause of the reported issue I would ask you to provide me with the couple details:
1. I noticed you use Aspose.Word version 9.4.0.0, is it correct? Did you try to reproduce issue with version 9.5.0.0 (the latest one)? If not yet, please try this (probably you need upload it).
2. How did you generate your pdf file: using some input template file or generating content programmatically? If you use template please attach it here.
3. Probably the issue’s appearance is caused by using image in header of your document. The similar issue was reported exactly for 9.4.0.0 version and has been fixed in 9.5.0.0. Try to exclude that image upon pdf generation. If problem is gone you definitely should use the latest release.