Swirly Music asks composers submit scores in PDF format, and that each score be compiled into one PDF file. This means you may need to assemble separate files, like a cover and any front matter, and the music pages themselves.
On a Mac, this is extremely easy to do using the Preview app that is native to the Mac OS X operating system. This video tutorial demonstrates how to do this exactly, with a bonus of showing how, on a Mac, any file that can be printed can also be swiftly converted to PDF format.
There are various utilities for Windows that may offer the same. Swirly Music does not wish to endorse any one product, and recommends following this search to see what’s available.