Most pdfs are compressed. A uncompressed pdf is a plain text file that is human readable (but your eyes will blead from the endless vector graphics tokens). There are some cmd line tools which can compress/uncompress and defragment/progressive download your pdf file. The pdf format allows stream generation with a server creating a pdf as a nonseekable stream, the allocation table goes on the end, if a token/block isn't in the final allocation table it is effectively free space but its still wastes space in the pdf file. A pdf file's tree allows reference looping BTW.