The xref table contains pointers to all the objects included in the PDF file. It identifies how many objects are in the table, where the object begins (the offset), and its length in bytes.

The xref table starts with the keyword xref.

Following this is a number of cross-reference subsections. The first line of each subsection consists of the object number of the first object, a space character, and the number of entries in this section. The objects in each subsection must be contiguous.

Following this are one line entries indicating the offset in bytes of the start of each indirect object. Each line must be exactly 20 bytes long including the EOL marker. The exact format is a 10 digit offset of the object in the stream, followed by space, a five digit generation number, space, n or f, CRLF. n indicates this entry is in use. f indicates this entry is free.

The first entry in the table (object number 0) shall always be free and shall have a generation number of 65,535.

xref
0 8
0000000000 65535 f
0000000015 00000 n
0000000064 00000 n
0000000123 00000 n
0000000196 00000 n
0000000372 00000 n
0000004655 00000 n
0000004675 00000 n

The first line indicates this xref subsection is for objects 0 though 7.