Open \gs\config\galleryserverpro.config and look for the HTML template for PDF files:
Code:<mediaObject mimeType="application/pdf">
<browsers>
<browser id="default"
htmlOutput="<p><a href="{MediaObjectUrl}">Enlarge PDF to fit browser window</a></p><iframe src="{MediaObjectUrl}" frameborder="0" style="width:680px;height:600px;border:1px solid #000;"></iframe>" />
</browsers>
</mediaObject>
Change the width and height to whatever you want.
Roger Martin
Creator and Lead Developer of Gallery Server Pro