By definition, pH=-log_10[H_3O^+]. And the use of the logarithmic function dates back to pre-electronic calculator days, when students, and engineers, and scientists, used logarithmic tables for more complex calculations, the which a modern calculator, available for a dollar or so, would EAT today....
For a strong acid, say HCl at MAXIMUM concentration, approx. 10.6*mol*L^-1, which is conceived to ionize completely in aqueous solution, we gots...
HCl(aq) + H_2O(l) rarr H_3O^+ +Cl^-
Now here, [H_3O^+]=10.6*mol*L^-1....
And so pH=-log_10[H_3O^+]=-log_10{10.6}=-(+1.03)=-1.03..
And thus for stronger acid [H_3O^+] GIVE A MORE NEGATIVE pH....
For background...
Just to note that in aqueous solution under standard conditions, the ion product...
K_w=[H_3O^+][HO^-]=10^(-14)...
And we can take log_10 of both sides to give....
log_(10)K_w=log_(10)10^(-14)=log_10[H_3O^+]+log_10[HO^-].
And thus.... -14=log_(10)[H_3O^+]+log_(10)[HO^-]
Or.....
14=-log_(10)[H_3O^+]-log_(10)[HO^-]
14=underbrace(-log_10[H^+])_(pH)underbrace(-log_10[OH^-])_(pOH)
14=pH+pOH
By definition, -log_10[H^+]=pH, -log_10[HO^-]=pOH.