The zlib-flate program standard from input and writes to standard output either compressing or compressing its input using raw zlib compression. This program is provided primarily as a debugging tool. It can be used to uncompress or compress raw PDF streams.
This program should not be used as a general purpose compression tool. Use something like gzip(1) instead.
For details about qpdf, please see the qpdf manual, which can be found in /usr/share/doc/qpdf/qpdf-manual.html or /usr/share/doc/qpdf/qpdf-manual.pdf.