Doc: pdf illustration

site/dev/design/pdftheory.md

 PDF Theory of Operation
 =======================
 
 <!--
 PRE-GIT DOCUMENT VERSION HISTORY
     2012-06-25 Steve VanDeBogart
                * Original version
     2015-01-14 Hal Canary.
                * Add section "Using the PDF backend"
                * Markdown formatting
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     draw_content(&canvas);
 
     SkPDFDocument doc;
     doc.appendPage(dev);
     doc.emitPDF(&pdf_stream);
 
 <a name="PDF_Objects_and_Document_Structure"></a>
 PDF Objects and Document Structure
 ----------------------------------
 
+![PDF Logical Document Structure](PdfLogicalDocumentStructure.png)
+
 **Background**: The PDF file format has a header, a set of objects and
 then a footer that contains a table of contents for all of the objects
 in the document (the cross-reference table). The table of contents
 lists the specific byte position for each object. The objects may have
 references to other objects and the ASCII size of those references is
 dependent on the object number assigned to the referenced object;
 therefore we can’t calculate the table of contents until the size of
 objects is known, which requires assignment of object
 numbers.
 
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 *   [issue 244](https://bug.skia.org/244)
     Mostly, only TTF fonts are directly supported.  (User metrics 
     show that almost all fonts are truetype.
 *   [issue 260](https://bug.skia.org/260)
     Page rotation is accomplished by specifying a different
     size page instead of including the appropriate rotation
     annotation.
 
 * * *