A primary lysosome is a membrane bounded sac that buds from the golgi apparatus.
A primary lysosome contains many enzymes (collectively called acid hydrolases) that are synthesized on the RER and sorted in the golgi.
The membrane of the lysosome has intergral proteins called hydrogen ion ATPases (proton pumps). These protein are involved in an active transport process whereby protons are transported into the lumen of lysosome from the cytosol of the cell. Since protons are being transported against their concentration gradient the proton pump is an ATPase. The relatively high concentraction of protons in the lumen of the lysosome results in a pH of about 5.0 in the lumen of the lysosome.
The "acid hydrolases" in the lysosome work best at this acidic (5.0) pH.