This is nothing new, but if you ever needed a simple way to convert a document from CHM format into PDF format on Linux, here's an easy way to do it with a fairly good result. You need to make sure that chmlib and htmldoc is installed.
On Debian you can get them with:
# apt-get install libchm-bin htmldoc
Next, extract the contents from the CHM file:
$ extract_chmlib doc.chm DEST_DIR
Where DEST_DIR is the destination directory. If the directory doesn't exist, it will be created.
In order to create a single PDF file of A4 size you can use the following command:
$ htmldoc --size a4 --webpage -f doc.pdf DEST_DIR/*.html