MarkT:
I would use current limiting resistors on the segment lines, not the digit lines, and add switching transistors
for the digit lines so they can carry enough current - an Arduino pin can drive one segment without
overload, but probably not 7, unless they are small low current displays.
Yup, you are right, adding transistor would be probably the safest way. I think that time, i was more focusing on performing multiplexing the 7 segment display, so i did not care much on the hardware part.