When describing the general form of a command, rather than a particular example, square brackets  are used to enclose optional parts, and angled braces <> to enclose general descriptions which may be substituted for particular instances. For example,
describes a command line which requires an operation and, optionally, a range. An exception is that we use in.pdf and out.pdf instead of <input file> and <output file> to reduce verbosity. Under Microsoft Windows, type cpdf.exe instead of cpdf.