Footer trap in a A4 PDF

[Matteo Bini]

Hello groffers!
I'm writing some documents that will be exported in PDF with A4 paper
size. I'm using my own macro package.

My macro package is very very simple: it's just for defining paragraphs
and titles style. Then I've decided to add an empty footer, which is
more like a bottom margin for my pages.

I've defined a macro called FO.
.de FO
footer
.  bp
..

I've set the page length to 29.7c (A4).
.pl 29.7c

I've set up a trap at -2c with my macro FO.
.wh -2c FO

This is how I output to PDF:
groff -K utf8 -m mymacro -M . -P -pa4 -T pdf in.roff > out.pdf

I would expect the word "footer" to pop up at -2c from the page bottom,
then a page break. Footer does pop up, and there's the page break too,
however it does not show up at -2c from the bottom. It's lower.

What am I doing wrong?

I'm quite new to groff, as well as to this mailing list, so, please, be
gentle ;)

--
Matteo Bini