The answer lies within your post:
This is a textbox, not a field.
Make it a field instead of a check box. This can be a field with global storage specified in field options or a field in a related table linked in with the X operator so that the same record matches to all records in your PDF report.
Perfect. This works very well.
[I have made a new window in which I can define the title for the header of the report/PDF.]