There are many malformed PDF files in existence, including many produced by otherwise-reputable applications. cpdf attempts to correct these problems silently.
Grossly malformed files will be reconstructed. The reconstruction
progress is shown on stderr (Standard Error):
Sometimes files can be technically well-formed but use inefficient PDF constructs. If you are sure the input files you are using are impeccably formed, the -fast option added to the command line (or, if using AND, to each section of the command line). This will use certain shortcuts which speed up processing, but would fail on badly-produced files.
The -fast option may be used with:
If problems occur, refrain from using -fast.