The 1859 paper by Bernhard Riemann which introduced the zeta-function and linked it to the Prime Number distribution is only 9 pages and quite approachable. It inspired much of analytic number theory:
"One now finds indeed approximately this number
of real roots within these limits, and it is very probable that all roots are
real. Certainly one would wish for a stricter proof here; I have meanwhile
temporarily put aside the search for this after some fleeting futile attempts,
as it appears unnecessary for the next objective of my investigation"
https://en.wikipedia.org/wiki/On_the_Number_of_Primes_Less_T...
It also contains his famous conjecture:
"One now finds indeed approximately this number of real roots within these limits, and it is very probable that all roots are real. Certainly one would wish for a stricter proof here; I have meanwhile temporarily put aside the search for this after some fleeting futile attempts, as it appears unnecessary for the next objective of my investigation"