PDF 1.5 introduced a new mechanism for storing objects to save space: object streams. by default, cpdf will preserve object streams in input files, creating no more. To prevent the retention of existing object streams, use -no-preserve-objstm:
cpdf -no-preserve-objstm in.pdf -o out.pdf
Write the file in.pdf to out.pdf, removing any object streams.
To create new object streams if none exist, or augment the existing ones, use -create-objstm:
cpdf -create-objstm in.pdf -o out.pdf
Write the file in.pdf to out.pdf, preserving any existing object streams, and creating any new ones for new objects which have been added.
To create wholly new object streams, use both options together:
cpdf -create-objstm -no-preserve-objstm in.pdf -o out.pdf
Write the file in.pdf to out.pdf with wholly new object streams.
Files written with object streams will be set to PDF 1.5 or higher, unless -keep-version is used (see above).
Object streams are always removed prior to linearization.