How does pH affect the spontaneity of biochemical processes?
Note that pH effect is reaction specific.
If a reaction is e.g. $\ce{A + B + H+ &lt;=&gt; C + D+}$ , or $\ce{A + B- &lt;=&gt; C + D + OH−}$,
then lowering pH supports the spontaneity.
If OTOH, the reaction is $\ce{A + B + OH-  &lt;=&gt; C + D−}$ , or $\ce{A + B+ &lt;=&gt; C + D + H+}$,
then lowering pH goes against the spontaneity.
It can be generalized by applying the Le Chatelier's principle, saying systems adapts to external changes by such equilibrium shifts that partially decreases the system chage caused by the external factor.
Lowering pH supports spontaneity of reactions consuming H+ and vice versa.
Increasing temperature prefers endothermic reactions consuming heat and vice versa.
Increasing pressure prefers reactions decreasing the total molar amount of gaseous components and vice versa.

Note that biological systems are much more complex than chemical systems.
Lowering pH may support thermodynamic spontaneity of some particular reaction. But an enzyme catalyzing its kinetic rate may start loosing its activity, get denatured, cells may stop producing the enzym, cells may stop overall life functions, or may die.