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Earlier versions of Publisher do not support saving as PDF. Use the method above if you're using Publisher 2003 or earlier. You may be asked to select the location before proceeding. This will allow you to save the file in PDF format. You may need to install the add-in from Microsoft available here for Publisher 2007. Publisher allows you to optimize your document for PDF format.  The Publish Options window will allow you to adjust the picture resolution. Click "Print Options" to adjust the document for printing. Choose a location and save the file in PDF format. You'll now be able to open that PDF in any program that supports the format.

Summary:
Open your PUB file in Publisher 2007 or later. Click the File tab or Office button and select "Save As." Click the "Save as type" menu and select "PDF (*.pdf)." Click "Options" to optimize your document for PDF (optional). Save the file.