Your original post refers to "a multi-page doc", so let's say you want
to do this in one particular Word document (not a whole bunch of Word
documents in one or more Windows folders). You don't say anything about
section breaks in this one particular Word document, so let's say the
Word document does not contain any section breaks. You don't say how
Word is supposed to know what the one project number is, so let's say
you don't mind typing the project number once, but you don't want to
type the project number on every page. You don't say what version of
Word you have, so let's say you have Word 2003 or earlier.
So let's say you use Word 2003 or earlier to open this one particular
Word document which contains no section breaks, click on View | Header
and Footer, press the Down arrow key until the cursor appears in the
footer, type the project number into the footer, and click on Close.
The footer you just created should now appear on all pages of this one
particular Word document.