can't find Page Tree Nodes？

I have read the pdf reference 1.6 carefully,i have decompress an decode the cross-reference stream.then i try to get the page tree nodes,i can't find it in a pdf of version 1.6

for example:

I get the Document Catalog,find the pages is :19 0 R ,but i can't find 19 obj in the pdf,wo can help me?

28 0 obj

<</Metadata 4 0 R/Names 38 0 R/Outlines 8 0 R/Pages 19 0 R/SpiderInfo 25 0 R/StructTreeRoot 13 0 R/Type/Catalog>>

endobj