I was able to download and transfer the proposal document to a Mini vMac emulator, set the Finder's type and creator to those of a Microsoft Word 5 document i.e. respectively WDBN and MSWD, and finally open the document with Microsoft Word 5 for Mac to export it as a RTF document.

Here you have it: https://neko.melomac.net/tmp/proposal.rtf

I certainly agree opening a document from this Macintosh era should be, by far, easier than the process I detailed below, but this is how it is ¯\_(ツ)_/¯