By definition, the resolution, DPI, is dots per inch. If you are generating a PDF at 8.5x11 or 36x48 with 300 DPI, the resolution is the same on both sizes. Just because the larger sheet has more dots doesn't mean it is a higher resolution. It's how many dots are on a line one inch long.
Now, if you take that 8.5 PDF at 300 DPI and print it on 36x48 the resolution will get lower because you have the same number of dots but more inches. Conversly, a 36x48 PDF at 300 DPI printed on 8.5x11 will have a higher resolution.