QPDF is a program that can be used to linearize (web-optimize),
encrypt (password-protect), decrypt, and inspect PDF files from the
command-line.  It does these and other structural, content-preserving
transformations on PDF files, reading a PDF file as input and
creating a new one as output.  It also provides many useful
capabilities to developers of PDF-producing software or for people
who just want to look at the innards of a PDF file to learn more
about how they work.

QPDF understands PDF files that use compressed object streams
(supported by newer PDF applications) and can convert such files into
those that can be read with older viewers.  It can also be used for
checking PDF files for structural errors, inspecting stream contents,
or extracting objects from PDF files.  QPDF is not PDF content
creation or viewing software -- it does not have the capability to
create PDF files from scratch or to display PDF files.