Every boidly fluid has different optimal pH ranges. The blood will be slightly alkaline 7.365, saliva and urine should fluctuate around 6.5 - 7 depending on timing.
However, we need a highly acidic environment in the stomach in order to sterilize our food and digest it properly. Usually around pH 1-3 would be ideal when eating. You won't be able to measure stomach contents pH with a pH strip.
When the stomach has proper acid levels you will digest food properly ionizing minerals and allowing absorbption of other nutrients which will regulate proper pH levels of other tissues.
Proper levels of stomach acid will trigger the gallbladder to release bile when you eat fat. This will emulsify the fat and allow lipase enzymes from pancreas to break it down and allow for fat absorption.
Lemon and limes are an acidic fruit (means low pH). When eaten they provide plenty of minerals which may have a general alkalizing effect in the body. The citric acid in the citrus fruits may lower the pH in the gut initially which may be helpful in the digestion.
However stomach acid is hydrochloric acid so the supplementing with Betaine HCl with meals may have a better impact in digestion.
Niacin can help you make more stomach acid naturally.