Set a fine-mesh sieve over a bowl and line it with several layers of cheesecloth or a nut-milk bag.
In a high-speed blender, combine the pumpkin seeds, water, and salt and puree on high speed for 2 minutes, until you see no specks.
Pour into a medium saucepan and cook over medium heat, stirring occasionally with a silicone spatula—and being sure to scrape the bottom of the pan to avoid scorching—until the mixture starts to gently bubble, 10 to 15 minutes. You should see the mixture start to form small curds.
Stir in the lemon juice and continue gently stirring, being careful not to break up the curds, until they separate a little more from the liquid, 2 to 3 minutes.
Spoon or pour the mixture into the cheesecloth-lined sieve or the nut-milk bag, smooth the top, and close the cheesecloth, layering it on top as neatly and tightly as possible.
Set a small smooth-bottomed plate on the cheesecloth and top it with a small can of beans or tomatoes to gently press about 20 minutes, until the “whey” has mostly stopped dripping out. (If you have a tofu-making press, that works well here, too, but you might need to work in two batches.)
Leaving the tofu in the cheesecloth or nut-milk bag, transfer it to an airtight container and refrigerate until firm, at least 2 hours. (You can store it like this for up to 1 week.)