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

printing/print_job_constants.h

 // 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.
 
 #ifndef PRINTING_PRINT_JOB_CONSTANTS_H_
 #define PRINTING_PRINT_JOB_CONSTANTS_H_
 
 #include "printing/printing_export.h"
 
 namespace printing {
@@ skipping 24 matching lines @@
 PRINTING_EXPORT extern const char kSettingLandscape[];
 PRINTING_EXPORT extern const char kSettingMarginBottom[];
 PRINTING_EXPORT extern const char kSettingMarginLeft[];
 PRINTING_EXPORT extern const char kSettingMarginRight[];
 PRINTING_EXPORT extern const char kSettingMarginTop[];
 PRINTING_EXPORT extern const char kSettingMarginsCustom[];
 PRINTING_EXPORT extern const char kSettingMarginsType[];
 PRINTING_EXPORT extern const char kSettingPageRange[];
 PRINTING_EXPORT extern const char kSettingPageRangeFrom[];
 PRINTING_EXPORT extern const char kSettingPageRangeTo[];
+PRINTING_EXPORT extern const char kSettingPreviewModifiable[];
 PRINTING_EXPORT extern const char kSettingPrinterName[];
 PRINTING_EXPORT extern const char kSettingPrintToPDF[];
 
 PRINTING_EXPORT extern const int FIRST_PAGE_INDEX;
 PRINTING_EXPORT extern const int COMPLETE_PREVIEW_DOCUMENT_INDEX;
 
 #if defined (USE_CUPS)
 // Printer color models
 PRINTING_EXPORT extern const char kBlack[];
 PRINTING_EXPORT extern const char kCMYK[];
@@ skipping 62 matching lines @@
 enum MarginType {
   DEFAULT_MARGINS,  // Default varies depending on headers being enabled or not
   NO_MARGINS,
   CUSTOM_MARGINS,
   PRINTABLE_AREA_MARGINS,
 };
 
 }  // namespace printing
 
 #endif  // PRINTING_PRINT_JOB_CONSTANTS_H_