This happens because the Print Quality is set to High by default in PowerPoint---which is much higher than what is typically needed. Most uses will work using the "Low" setting, and "Medium" is often as high as the Print Quality needs to be set. Otherwise, the PDF becomes so large that it is actually difficult for students to download.
You can set the Print Quality of the PDF to High, Medium, or Low—depending on your need. By default, the print quality is set to High—which is the optimal choice for printing a PDF. Follow these instructions to set the print quality of the PDF:
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Click the PowerPoint menu > Preferences.
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In the PowerPoint Preferences dialog box, click General.
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Under Print Quality (Paper/PDF), set the Desired Quality to an option of your choice.
Try "Low" first, to see if the Print Quality is adequate. If not, you can bump it up to "Medium"---but, again, for most applications, "Low" will suffice.