Hi,
You say "input" because you think of current flow.
But indeed it is an "output".
Think about "information" flow. The microcontroller sends information to the display: which digit is "ON".
Input: there is never a significant current flow of interest. (When pullup is enabled, there maybe current flow, but just to prevent the signal from floating and reduce external part count).
Output: the microcontroller forces HIGH or LOW to enable current flow. Often to drive a load.
But the drive current capability is limited.
Read the datasheet about
* output drive capability per pin (not the "absolute maximum rating")
* power supply pin current capability
And like always: draw a sketch for us ... and for you. It doesn't need to be perfect, paper and pencil is OK.
Schematics are far easier to read than a lot of text. And it helps to avoid misunderstandings.
Klaus