We can see that the PDF document still contains the original header, body, cross-reference table and the trailer. Additionally, there are also other body, cross-reference and trailer sections that were added to the PDF document.
The additional cross-reference sections will contain only the entries for objects that have been changed, replaced or deleted. In PDF versions 1. Upon opening this PDF document it looks as shown below:. Figure 4: PDF document sample. Figure 5: Cross-reference and trailer sections.
The cross-reference section has been reduced for clarity. The cross-reference section contains one subsection that itself contains objects. The trailer section starts at byte offset , includes objects where the root element points to object and the info element points to object The PDF document contains eight basic types of objects described below. These types are: booleans, numbers, strings, names, arrays, dictionaries, streams and the null object.
Objects may be labeled so that they can be referenced by other objects. A labeled object is also called an indirect object. There are two keywords: true and false that represent the boolean values. There are two types of numbers in a PDF document: integer and real. An integer consists of one or more digits optionally preceded by a plus or minus sign. An example of integer objects may be seen below:.
The real value can be represented with one or more digits, with an optional sign and a leading, trailing or embedded decimal point a period. An example of real numbers can be seen below:. There is a limitation of the length of the name element, which may be only bytes long. When writing a name, a slash must be used to introduce a name; the slash is not part of the name but is a prefix indicating that what follows is a sequence of characters representing the name.
If we want to use whitespace or any other special character as part of the name, it must be encoded with two-digit hexadecimal notation. Figure 6: PDF names source. Strings in a PDF document are represented as a series of bytes surrounded by parenthesis or angle brackets, but can be a maximum of bytes long.
Any character may be represented by ASCII representation, and alternatively with octal or hexadecimal representations. Octal representation requires the character to be written in the form ddd, where ddd is an octal number. An example of representing a string embedded in parentheses can be seen below:.
We can also use special well-known characters when representing a string. Those are: n for new line, r for carriage return, t for horizontal tabulator, b for backspace, f for form feed, for left parenthesis, for right parenthesis and for backslash.
Arrays in PDF documents are represented as a sequence of PDF objects, which may be of different types and enclosed in square brackets. This is why an array in a PDF document can hold any object types, like numbers, strings, dictionaries and even other arrays. An array may also have zero elements. An array is presented with a square bracket. An example of an array is presented below:. The key must be the name object, whereas the value can be any object, including another dictionary.
The maximum number of entries in a dictionary is entries. A stream object is represented by a sequence of bytes and may be unlimited in length, which is why images and other big data blocks are usually represented as streams. A stream object is represented by a dictionary object followed by the keywords stream followed by newline and endstream. The stream dictionary specifies the exact number of bytes of the stream.
After the data there should be a newline and the endstream keyword. Common keywords used in all stream dictionaries are the following note that the Length entry is mandatory :. The stream data in the object stream will contain N pairs of integers, where the first integer represents the object number and the second integer represents the offset in the decoded stream of that object. The First entry in the dictionary identifies the first object in the object stream.
In PDF 1. Each cross-reference stream contains the information equivalent to the cross-reference table and trailer. First of all, we must know that any object in a PDF document can be labeled as an indirect object. This gives the object a unique object identifier, which other objects can use to reference the indirect object.
By declaring an object an indirect object, we are able to use it in the PDF document cross-reference table and reuse it by any page, dictionary and so on in the document. Since every indirect object has its own entry in the cross-reference table, the indirect objects may be accessed very quickly. The object identifier of the indirect object consists of two parts; the first part is an object number of the current indirect object.
These example sentences are selected automatically from various online news sources to reflect current usage of the word 'pdf. Send us feedback. Our team at The Usage has selected the best MacBooks. See more words from the same year. Accessed 14 Jan.
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As we mentioned earlier, PDFs are primarily meant for viewing, not editing. However, there may be times when you encounter a PDF that allows you to enter certain information, like name and address. These PDFs use a special feature called form fields , which allow you to type new information into the document and save your changes. For example, this PDF contains several form fields for entering your name, address, and other information. Programs that support OCR allow you to digitally scan the document for text and then edit or delete it.
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