There is plenty of ISF file types available and it is likely that at least some of them can be exported as PDFs. Try to look for a dedicated PDF export function in programs that open them, or you can always try to print to a PDF file with a virtual printer installed. This should be possible in any program that can both open (view) and print ISF files.
Convert Inspiration diagram to Adobe Portable document format.
We found one software available in the database for this conversion.
Microsoft Windows software - convert isf to pdf on Windows
A ultimate brainstorming software tool
Convert from isf 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.