Cross-reference table (xref)
Which contains a reference to all objects in the document. Cross-reference list is intended to allow a random access file objects, so we do not need to read the entire PDF document to locate specific objects. Each object is represented by a cross-reference entries in the table. (This table contains all indirect physical objects in a PDF file offset address; in the table may be a single file, a plurality may be a plurality of cross-reference table typically occurs in two situations: First, the incremental saving. Second, linearized.)
example
xref // 0 for the first object ID, an object comprising
0 1
0000000023 65535 f // offset 23 always contains a generation number represented by the entry of the idle f 65535
3 1
0000025324 00000 n
21 1
0000025518 00002 n // from the object is in use become idle, n becomes f, increasing the number of modification 1
22 1
0000025632 00000 n
23 1
0000000024 00001 f // becomes idle by the objects in use, but becomes f n, without increasing the number of modified
24 1
0000000000 00001 f
36 1
0000026900 00000 n // cross-reference table of the number of the last modification is always 0