SkDocument_PDF does somthing with the trimBox parameter.

src/doc/SkDocument_PDF.cpp

 /*
  * Copyright 2013 Google Inc.
  *
  * Use of this source code is governed by a BSD-style license that can be
  * found in the LICENSE file.
  */
 
 #include "SkDocument.h"
 #include "SkPDFDocument.h"
 #include "SkPDFDevice.h"
@@ skipping 26 matching lines @@
         mediaBoxSize.set(SkScalarRoundToInt(width), SkScalarRoundToInt(height));
 
         fDevice = SkNEW_ARGS(SkPDFDevice, (mediaBoxSize, mediaBoxSize, SkMatrix::I()));
         if (fEncoder) {
             fDevice->setDCTEncoder(fEncoder);
         }
         if (fRasterDpi != 0) {
             fDevice->setRasterDpi(fRasterDpi);
         }
         fCanvas = SkNEW_ARGS(SkCanvas, (fDevice));
+        fCanvas->translate(trimBox.x(), trimBox.y());
+        fCanvas->clipRect(SkRect::MakeWH(trimBox.width(), trimBox.height()));

[vandebo (ex-Chrome), 2014/11/11 15:44:53]

Do headers and footers still work with this approach?

         return fCanvas;
     }
 
     virtual void onEndPage() SK_OVERRIDE {
         SkASSERT(fCanvas);
         SkASSERT(fDevice);
 
         fCanvas->flush();
         fDoc->appendPage(fDevice);
 
@@ skipping 42 matching lines @@
 SkDocument* SkDocument::CreatePDF(const char path[],
                                   SkPicture::EncodeBitmap enc,
                                   SkScalar dpi) {
     SkFILEWStream* stream = SkNEW_ARGS(SkFILEWStream, (path));
     if (!stream->isValid()) {
         SkDELETE(stream);
         return NULL;
     }
     return SkNEW_ARGS(SkDocument_PDF, (stream, delete_wstream, enc, dpi));
 }