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Introduction to PDF file format

In this section, we present the basic structure of a PDF document. For details, please refer to the PDF Reference Manual. Below is a listing of a very simple PDF document. It displays a "Hello World" string on a single page.

0000    %PDF-1.4 0001    1 0 obj <<
0002      /Parent 5 0 R
0003      /Resources 3 0 R
0004      /Contents 2 0 R
0005    >>
0006    endobj
0007    2 0 obj
0008    <<
0009      /Length 51
0010    >>
0011    stream
0012      BT
0013      /F1 24 Tf
0014      1 0 0 1 260 330 Tm
0015      (Hello World)Tj
0016      ET
0017    endstream
0018    endobj
0019    3 0 obj
0020    <<
0021      /ProcSet \[/PDF/Text\]
0022      /Font <</F1 4 0 R >>
0023    >>
0024    endobj
0025    4 0 obj <<
0026      /Type /Font
0027      /Subtype /Type1
0028      /Name /F1
0029      /BaseFont/Helvetica
0030    >>
0031    endobj
0032    5 0 obj
0033    <<
0034      /Type /Pages
0035      /Kids \[ 1 0 R \]
0036      /Count 1
0037      /MediaBox \[0 0 612 714\]
0038    >>
0039    endobj
0040    6 0 obj
0041    <<
0042      /Type /Catalog
0043      /Pages 5 0 R
0044    >>
0045    endobj 0046    xref
0047    0 7
0048    0000000000 65535 f
0049    0000000009 00000 n
0050    0000000103 00000 n
0051    0000000204 00000 n
0052    0000000275 00000 n
0053    0000000361 00000 n
0054    0000000452 00000 n
0055    trailer
0056    <<
0057      /Size 7
0058      /Root 6 0 R
0059    >>
0060    startxref
0061    532

A PDF document consists of four sections:

  • A one-line header identifying the version of the PDF specification to which the file conforms (Line 0). In the above sample, the header string is "%PDF-1.4". It identifies this file as a PDF document adhering to the 1.4 specification.
  • A body containing the objects that make up the document contained in the file (Lines 1-45). Our sample file shows 6 objects each beginning with "obj" and ending with "endobj". Each object has its own number and a zero. The zero is the revision level (also known as the generation number) because PDF allows updates to the file to be made without rewriting the entire file.
  • A cross-reference table containing information about the indirect objects in the file (Lines 46-54). The cross-reference table in our sample notes that it contains 7 entries; a dummy for object zero and one for each of the 6 objects. The table maps implicit object index into a byte offset from the beginning of the file to the location where the object is located. For example, Object 1 is represented first indicating that it begins at byte 9; Object 3 is represented with the fourth entry indicating that it is located at byte 204 in the file. etc.
  • A trailer giving the location of the cross-reference table and of certain special objects within the body of the file (Lines 55-61).

Note that objects refer to each other using a notation like "5 0 R". The "R" stands for reference and uses the two preceding numbers to name a specific object and revision.

Therefore, the file body consists of a collection of objects, each object potentially referencing any or all objects, including itself. This set of nodes and directed references constitutes a graph. We could represent the "Hello World" sample file using the following abstract graph representation.

Each object in the graph is represented with an ellipse and each object cross reference is represented with an arrow.

Each PDF document must have a "Root" node. It must reference a "Catalog" node which must reference a "Pages" node. The "Pages" node further branches and points to each of the pages in the document. Note that a "Pages" node points to a group of pages whereas a "Page" node represents a single page.

The "Page" node references the page's "Contents" and the page's "Resources". The resource dictionary, in turn, references the "Fonts" used on the page. The resource dictionary can reference many other resource types, including Color Spaces, Patterns, Shadings, Images, Forms, and more. The page contents stream contains markup operators used to draw the page.

Each PDF document uses this basic object structure to represent a PDF document.

Before going into details of Apryse SDK SDF/COS object model, we should review the basics. For a detailed description of the SDF syntax and semantics, please refer to Chapter 3 (Syntax) of the PDF Reference Manual.

In PDF there are five atomic objects:

Object TypeDescriptionSamples
NumberPDF provides two types of numeric object: integer and real.1.03 612
BoolBoolean objects are identified by the keywords true and false.true false
NameA name object is an atomic symbol uniquely defined by a sequence of characters. Names always begin with "/" and can contain letters and numbers and a few special characters./Font /Info /PDFNet
StringStrings are sequences of bytes enclosed in "(" and ")"(Hello World!)
NullThe null object has a type and value that are unequal to those of any other object. Usually it refers to a missing object.null

Also, there are three compound objects:

Object TypeDescriptionSamples
ArrayAn array object is a one-dimensional collection of objects arranged sequentially. Unlike arrays in typical computer languages, PDF arrays may be heterogeneous; that is, an array's elements may be any combination of numbers, strings, dictionaries, or any other objects, including other arrays.[ true /Name ]
DictionaryA dictionary object is a map containing pairs of objects, known as the dictionary's entries. The first element of each entry is the key and the second element is the value. The key must be a name. The value can be any kind of object, including another dictionary.<</key /value >>
StreamA stream is essentially a dictionary followed by a sequence of bytes. PDF streams are always indirect objects and thus always may be shared.1 0 obj << /Length 144 >> stream ........... endstream endobj

Objects can be arbitrarily nested using the dictionary and array compounding operations.

All of the objects in the above tables are "direct objects" because they are not surrounded by "obj" and "endobj" keywords. The body of the PDF document is actually made up of a sequence of "indirect objects". An indirect object is created by taking a single direct object (whether it be atomic or compound) and enclosing it with the "nm obj" and "endobj" keywords (where n and m are non-negative integers).

Note that, because indirect objects are numbered and can be referenced by other objects, they can be shared — that is, referenced by more than one other object. However, since direct objects are not numbered, they can't be shared.

In the above PDF example, the object "3 0 obj" is an indirect object because the "obj" and "endobj" keywords wrap a dictionary object containing two entries.

3 0 obj
  /ProcSet \[/PDF /Text\]
  /Font << /F1 4 0 R >>

The "ProcSet" key is mapped to an array which is a direct object containing atomic direct objects. In a similar way, the "Font" key is mapped to a direct dictionary. On the other hand, "F1" in the inner dictionary is mapped to an indirect object with object number 4 and generation number 0. Because the "Font" key points to an indirect object, the same font resource can be shared across many different pages.

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