We're happy to give you some advice on this process. It's not entirely simple as there are a few potential roadblocks, but it's a very good idea to modernize your father's work while you can. You never know when an old Mac will stop working, and you don't want his work to remain stranded in non-portable file formats.
I'll go over the basics and you can let me know if you have any questions. Please be aware that Nisus Classic is no longer supported, so while I can give advice and help, you might have to troubleshoot on your own to overcome any problems you encounter.
Here are the issues involved in modernizing old Nisus Classic files:
1. Resource Forks
. These are part of the old Mac OS9 file system and were used by Nisus Classic to store all formatting information for your documents; resource forks don't really exist in OSX / macOS. It's very important that you don't lose any resource forks for your old files. A classic file without a resource fork will at best have no formatting or images, and at worst will be completed garbled, text and all. You will need to archive (zip) your classic files BEFORE you transfer them off your old Mac. Please see our FAQ topic on classic files
Once you transfer your old files (with resource forks) to a modern Mac you can use Nisus Writer Pro to import them. Nisus Writer can read the Nisus Classic file format and save new files using a standardized file format like RTF.
2. Legacy Fonts
. Assuming you transfer and import your old classic files, you might still be working with legacy fonts. The problem is that these fonts were designed before Unicode arrived and standardized text encodings. This means that the correct display of text on screen is undesirably tied to a particular font. If your font is not available on a modern Mac (or has otherwise changed), your text will display incorrectly or as complete gibberish. How big a problem this is depends on the fonts you used, and how many employed non-standard text encodings.
If you again consult the aforementioned FAQ on classic files
you will see you can maybe write a custom Nisus Writer Pro macro to help you convert your old text. Such a macro will modernized any garbled text using old fonts, so the characters are converted to proper Unicode characters.
3. Mac PICT Images
. If you have any images in your Classic files they are most likely in the Mac "PICT" image format. Although the current Mac system still supports PICT images, we've seen that support has degraded over the years. Your images may display correctly on screen, but they might also suffer minor (or major) display problems- or simply not display at all. It's highly unlikely Apple will fix these problems, as the PICT image format is dead. You should replace all your images using new image formats as soon as possible.
I hope that helps, and good luck!