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Not all of the recommended fixes appear to work for everyone.
Here is what worked for me, based on my own experiments together with the solution suggested on the page http://www.tug.org/pipermail/pdftex/2001-March/000450.html
As I am working with A4 portrait slides, my documents now begin with:
\documentclass[a4,portrait]{seminar} \usepackage[pdftex]{graphicx} \pdfcompresslevel=9 \pdfpagewidth=8.27 truein % A4 portrait \pdfpageheight=11.69 truein % A4 portrait \pdfhorigin=1truein % default value(?), but doesn't work without \pdfvorigin=1truein % default value(?), but doesn't work without \slideheight 23cm \slidewidth 17.5cm[an error occurred while processing this directive]