When this story first came up I got interested in learning about intaglio printing. We have probably all encountered examples of this impressive printing technique, most likely via invitations to weddings and other such events. One recognizes it by the feel of it to the fingers. The ink is raised above the surface of the paper making it positively palpable.

It's expensive, and looks and feels expensive.

This Wiki page tells all about it:

Intaglio (printmaking)
https://en.wikipedia.org/wiki/Intaglio_%28printmaking%29

This image shows how the lettering is in fact raised above the surface. The orange and yellow is the paper surface, the greenish color shows that the lettering is some number (10-15?) of micrometers (µm) above the paper. A micrometer is a millionth of a meter.

Micro-topography of an ordinary French post stamp (detail) showing the thickness of ink obtained by intaglio. The words "la Poste" appeared in white on red background and hence corresponds to areas with a lack of ink.

The Wiki article above describes the complexity of the process. The paper must be wet first. The plate must be carefully wiped after the ink is applied.

The difficulty of the process makes it very difficult for anyone to make forgeries of such documents.