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 $$ F(x) = \int f(x) \, $$
D. ???
(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.
I personally prefer the following notation for integration \[ \int f(x)\ dx \] I try to leave no room for ambiguity and to me this notation makes the integral clear, explicit and easily understood. Also the $dx$ can be read as "with respect to $x$".