The Rambam writes, as well as the Rema in Shulchan Aruch, that Ktav Ashurit, the special script that is reserved for Sifrei Torah, Tefillin, and Mezuzot, should not be used for devarim shel chol, mundane material. This is in order to show proper respect for the special and holy ktav of Ktav Ashurit.
This applies to any written or printed matter, including posters and invitations. Furthermore, it is advisable that even when writing words of Torah, but not a Sefer Torah, Tefillin, or Mezuzot, a script that looks similar to Ktav Ashurit should not be used.
The Rebbe suggested that the script be modified so that it should not look like the writing in a Sefer Torah. Similarly, someone wishing to print a fancy invitation, should refrain from using a font similar to Ktav Ashurit.