A file ending in the extension .dylib is
a dynamic library: it's a library that's loaded at runtime instead of at compile time. If you're familiar with DLLs from Windows or DSOs, it's more or less the same type of thing with a few twists. The Dynamic
Library Programming Topics section of the Mac OS X Developer Library covers all the details about the format and what you should be aware of.
libz.dylib is
the dynamic library for Zlib,
a general compression library. PDFs can (and usually do) use zlib to compress different aspects of the data contained within them, but accessing the PDF data at that level is pretty low-level, and higher-level libraries would abstract most of that type of
stuff.