Some current distributions of pdfTeX have a bug (feature?) that
using \magnification rescales the paper along with the
printing. The results can look very puzzling. This happens only with
pdf output, not with dvi.
The reason is that the pdf dimensions relating to the paper size
have been set in pt instead of truept. The
cure is simple enough: after \magnification
just insert something like this.
\pdfpagewidth=210.2truemm \pdfpageheight=297.3truemm % This is for A4 paper, use 8truein and 11truein for letter-size paper. \pdfhorigin=1truein \pdfvorigin=1truein \hsize=\pdfpagewidth \advance\hsize by -2truein \vsize=\pdfpageheight \advance\vsize by -2.1truein
There are some patches that will save you this trouble. Javier Bezos's fix is included in TeX Live. Ian Hutchinson offers another solution which is a small masterpiece of deviousness in itself. It needs to be read in full to be appreciated, but here is a sample, one worthy of any list of TeX Dirty Tricks.
\begingroup
\catcode`P=12 \catcode`T=12
\lowercase{\endgroup\def\convert@truept#1PT{#1truept}}