A. Year 1 Calculus Student Approach $$ F(x) = \int f(x') dx\, $$
B. Random math paper you find online approach $$ F(x) = \int dx f(x') \, $$
C. Spivak $$ F(x) = \int f(x) \, $$
(Edit) I'm asking because I read a post on Math Stackexchange that said something about dropping the dx is totally fine because it is not well defined. I'm curious as to what real mathematicians do.