Feel free to...

There's a new employee at your company. You've just introduced yourself. You offer to help this person.

If you have questions about anything, feel free to come by and ask, any time.

 feel free to (do something)

When you want to allow people to do something, you can say "feel free to ___." This is a formal phrase that you usually use with people you don't know very well, or with large groups:

Feel free to take one if you want it.

Feel free to call me with any questions.

This phrase is often used in business situations, with coworkers, employees, bosses, and clients.

If you have (any) questions, (do something)

"Having questions" means that you want to ask about something.

English speakers often talk about what someone can do if they "have questions":

If you have any questions, please contact us via our Facebook page.

If you have any questions, I'll be glad to answer them. 

come by (somewhere)

To "come by" somewhere means to visit a person or place. This phrase sounds friendly and casual:

Hey, why don't you come by my office on your way over there and I'll give you a tour.

We're having a party tonight. Come by if you're not busy.

I have to work, but maybe I'll come by after I finish.