The null object has a type and value that are unequal to those of any other object. There is only one object of type null, denoted by the keyword null.
Boolean object - PDF provides boolean objects identified by the keywords true and false. Boolean objects can be used as the values of array elements and dictionary entries.
PDF provides two types of numeric object: integer and real. Integer objects represent mathematical integers within a certain interval centered at 0. Real objects approximate mathematical real numbers, but with limited range and precision; they are typically represented in fixed-point, rather than floating-point, form. The range and precision of numbers are limited by the internal representations used in the machine on which the PDF viewer application is running.
A name object is an atomic symbol uniquely defined by a sequence of characters. Uniquely defined means that any two name objects made up of the same sequence of characters are identically the same object. Atomic means that a name has no internal structure; although it is defined by a sequence of characters, those characters are not “elements” of the name.
A string object consists of a series of bytes—unsigned integer values in the range 0 to 255. The string elements are not integer objects, but are stored in a more compact format. The length of a string is subject to an implementation limit.
An array object is a one-dimensional collection of objects arranged sequentially. Unlike arrays in many other 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. The number of elements in an array is subject to an implementation limit.
A dictionary object is an associative table 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 (unlike dictionary keys in Post-Script, which may be objects of any type). The value can be any kind of object, including another dictionary. A dictionary entry whose value is null is equivalent to an absent entry.
A stream object, like a string object, is a sequence of bytes. However, a PDF application can read a stream incrementally, while a string must be read in its entirety. Furthermore, a stream can be of unlimited length, whereas a string is subject to an implementation limit. For this reason, objects with potentially large amounts of data, such as images and page descriptions, are represented as streams.
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