When printing a PDF, query and use the print scaling disabled option.

chrome/browser/resources/print_preview/print_preview.js

 // Copyright (c) 2011 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 // require: cr/ui/print_preview_cloud.js
 
 var localStrings = new LocalStrings();
 
 // If useCloudPrint is true we attempt to connect to cloud print
 // and populate the list of printers with cloud print printers.
@@ skipping 338 matching lines @@
        'copies': copiesSettings.numberOfCopies,
        'collate': copiesSettings.isCollated(),
        'landscape': layoutSettings.isLandscape(),
        'color': colorSettings.colorMode,
        'printToPDF': printToPDF,
        'printWithCloudPrint': printWithCloudPrint,
        'isFirstRequest' : false,
        'headerFooterEnabled': headerFooterSettings.hasHeaderFooter(),
        'marginsType': marginSettings.selectedMarginsValue,
        'requestID': -1,
-       'generateDraftData': generateDraftData};
+       'generateDraftData': generateDraftData,
+       'previewModifiable': previewModifiable};
 
   if (marginSettings.isCustomMarginsSelected())
     settings['marginsCustom'] = marginSettings.customMargins;
 
   var printerList = $('printer-list');
   var selectedPrinter = printerList.selectedIndex;
   if (cloudprint.isCloudPrint(printerList.options[selectedPrinter])) {
     settings['cloudPrintID'] =
         printerList.options[selectedPrinter].value;
   }
@@ skipping 737 matching lines @@
 <include src="copies_settings.js"/>
 <include src="header_footer_settings.js"/>
 <include src="layout_settings.js"/>
 <include src="color_settings.js"/>
 <include src="margin_settings.js"/>
 <include src="margin_textbox.js"/>
 <include src="margin_line.js"/>
 <include src="margin_utils.js"/>
 <include src="margins_ui.js"/>
 <include src="preview_area.js"/>