When you view the .ssi web page in a web browser, it should be quite possible to export and save the output in a PDF document. Should be doable in every we browser that can processes .ssi files, so perhaps every modern one.
Convert HTML with server side includes to Adobe Portable document format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert ssi to pdf on Windows
A popular free web browser from Mozilla
Convert from ssi file format
The table with program actions contains information about what each program is capable of doing with their files. This may be a good pointer for further searches for certain file conversions and help you find the converter you are looking for if our search result is insufficient for you. However, it is far from perfect and may sometimes show results which are not really usable because of the nature of the software handles the files. So a conversion that appears as possible, just because of matched actions, may in fact not be possible.