Hi there, a PDF (Portable Document Format) is a non-editable format for print or reading using a "PDF Reader" or "PDF Viewer" (Adobe Acrobat, for example).
As this game is tagged in the "Physical Games - One you can play without devices (eg. board game, print & play)" category, you'll need to open the PDF and read the rules to find out how to play the game (I've included instructions at the bottom of this post for opening a PDF).
Once you've successfully opened the file, you'll see that it is a streamlined one page ruleset for a tabletop roleplaying game (TTRPG, a sub-category of the previously mentioned "Physical Games" category). Traditionally tabletop roleplaying games are played using your imagination, with dice rolls helping you to randomise some of the narrative. This game is best played with a small group of players, rather than alone, and requires one player to take the role of Games Master (GM) to help provide wider context to the narrative and adjudicate the outcomes of dice rolls. If you don't have anyone to play with already, there are usually plenty of local "Looking for games" or "LFG" groups online, or you could ask at your friendly local games store (FLGS).
Hope that clears everything up :)
How to open a PDF:
Find the PDF you want to open in your Files and double click to open. Select Adobe Acrobat (or whichever reader you downloaded) from the list of available options. If no list appears or the page opens in another application, you can right-click the file and select Open With to choose your PDF reader. Click Open.