If you buy Zork Anthology on Steam or GoG, it includes a copy of Planetfall. So that’s probably the easiest and most legitimate way to play this, I would guess?
Archive.org hosts a version of Planetfall for DOS, so if you want an authentic experience, that’s the way to go I assume: https://archive.org/details/msdos_Planetfall_1983
But there are many other options. As people might or might not be aware, Infocom games run in a virtual machine called Z-Machine. This is how they were able to release their games for many different platforms. And nowadays there are many modern implementations for modern platforms, so you can play these games in a more familiar or comfortable environment. IFWiki has a list of software programs you can use: https://www.ifwiki.org/List_of_Z-machine_interpreters
As for the game itself, there is a long history of people collecting and archiving different versions of Infocom games and source files. The most extensive collection available to the public that I know of is the one on eblong.com: https://eblong.com/infocom/ – here you can find different versions of Planetfall, both source and compiled to Z-Machine format, as well as many other games.
I wonder which version people are playing! I got the “final-dev” version of the game from eblong.com and am running it in the Frotz Z-Machine interpreter (https://davidgriffith.gitlab.io/frotz/)
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