X_JBIG2 label in viewer's upper left corner

Hi,
I'm working with a new set of pdfs (tax documents provided by the Public Office) and I've noticed a small label in the upper left corner of every page, which displays X JBIG2]
What does that mean? Can it be taken off? (knowing my customers they will start phoning me asking about this label).

Thanks in advance

Thank you Gui for your answer. I've tried the external tool but it uses a VC's runtime library that I don't have. Finally I have decided to optimize the PDFs with Acrobat, changing the monochrome compresion algorithm to CCIT 4.
The label is not displayed anymore.

Edorta