Add code snippet to set PDF magnification

Fix #692.

docs/tips-tricks.rst

@@ -1,10 +1,12 @@
-Tips & tricks
+Tips & Tricks
 =============
 
 This page presents some tips and tricks, mostly in the form of code snippets.
 
 .. note::
-	These tips are primarily sourced from the community. You too can share your tricks with the community, just open a PR! (If you do so, don't forget to make your code readable for the others and add some context :)).
+	These tips are primarily sourced from the community. You too can share your tricks with the community, just open a PR! (If you do so, don't forget to make your code readable for the others and add some context :).)
+
+
 
 Include header and footer of arbitrary complexity in a PDF
 ----------------------------------------------------------
@@ -217,3 +219,41 @@ Show me the code!
 .. note::
 
 	In the `CSS Generated Content for Paged Media Module <https://www.w3.org/TR/css-gcpm-3/>`_, the W3C proposed standards to support most expected features for print media. `Running elements <https://www.w3.org/TR/css-gcpm-3/#running-elements>`_ are the CSS compliant solution to this problem. See this `issue on the project <https://github.com/Kozea/WeasyPrint/issues/92>`_ for more details for a possible implementation.
+
+
+Edit the generated PDF using WeasyPrint's PDF editor
+----------------------------------------------------
+
+Why this snippet?
+.................
+
+You may want to edit the PDF generated by WeasyPrint, for example to add PDF features that are not supported by CSS properties.
+
+WeasyPrint includes a very simple and limited PDF editor that can be used in this case. This PDF editor only works with documents generated by WeasyPrint.
+
+In this example, we will set the magnification to "Fit page", so that the PDF size automatically fits in the PDF reader window when open.
+
+How to use this snippet?
+........................
+
+You can use the code below as a simple Python script. Change the URL you want to render and the path of the generated PDF to fit your needs.
+
+If you want to add other features, you will have to read the PDF specification!
+
+Show me the code!
+.................
+
+.. code-block:: python
+
+    from io import BytesIO
+    from weasyprint import HTML
+    from weasyprint.pdf import PDFFile, pdf_format
+
+    html = HTML('http://weasyprint.org/')
+    content = BytesIO(html.write_pdf())
+    pdf_file = PDFFile(content)
+    params = pdf_format('/OpenAction [0 /FitV null]')
+    pdf_file.extend_dict(pdf_file.catalog, params)
+    pdf_file.finish()
+    pdf = pdf_file.fileobj.getvalue()
+    open('/tmp/weasyprint.pdf', 'wb').write(pdf)