QPDF is a program that does structural, content-preserving
transformations on PDF files.  It could have been called something
like pdf-to-pdf.  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 offers many capabilities such as linearization (web
optimization), encrypt, and decription of PDF files.  Note that QPDF
does not have the capability to create PDF files from scratch; it is
only used to create PDF files with special characteristics starting
from other PDF files or to inspect or extract information from
existing PDF files.