While id Software famously released the Quake engine source code under the GPL (leading to wonders like Darkplaces and Ironwail ), the —those PAK files—remains the exclusive property of id Software (now owned by Microsoft/ZeniMax). You cannot legally download pak0.pak from a random Dropbox link.
You can verify with:
Thus, when you search for "quake 1 pak files download verified" , ethical results will point to or tools to extract from your own purchased copy . quake 1 pak files download verified
If you just want to test a source port, you can legally download the shareware version. This includes While id Software famously released the Quake engine