ppm is usually a molar ratio: 1 mole substance per 1x106 moles of total solution.
mg/L (milligrams/liter) is more often used for liquid solutions
pH is related to the molar concentration of the H+ ion in a solution in moles/liter. The pH value is the negative logarithm of the moles H+ concentration in a liter of the solution.
So we have several interacting concentration, volume and mass measures in play.
Let's assume a pH of 5 as our test case.
This means there are 1x10-5 moles of H+ in one liter of the solution. At this low concentration the mass of the solution will not be much different fom water, so the solution has mass of 1000 grams or 1,000,000 milligrams.
H+ has a molar mass of 1 gram/mole or 0.00001 gram per 10-5 mole.
So a weight ppm value of the H+ ion in the solution is 10-5 g/liter (or 1 million milligrams). Converting g to mg we arrive at the answer that a pH contains 10-2 ppmw of H+.
However if the ppm is to indicate the ppm value of the acid contributing the H+ ions it is necessary to change the molar mass of H+ to the molar mass of the acid (you have to know what acid is present).
Using H2SO4 as an example, the molar mass is 98 g/mole and each mole
contributes 2 moles of H+ to the solution. There would be (98/2)x 10-5 ppmw (or (49x 10-5 ppmw)of H2SO4 in the solution.