Can enzymes function at any pH?

No. They function best at the pH corresponding to their usual/intended environment. For example, pepsin, present in the stomach, which is highly acidic, functions best at acidic pH, while trypsin, secreted into the duodenum together with basic bicarbonate, functions best at moderately basic pH. This is true also within subcellular compartments: the optimal pH of lysosomal enzymes is acidic, matching the acidic proteolytic environment inside the lysosome. That said, most enzymes present in the cytosol (~neutral) and blood (~neutral) function best around neutral pH.