Q:

What is the meaning of "modest in scope"? 

this the context:

this paper was modest in scope. 

I think I understand it but I wanna be sure.  I think the meaning of that is:

the range of paper was limited. 

Am I right and if not, can you explain the meaning? 

A:

Yes, it does mean that the range of the paper was limited.  But by using the word modest instead of limited, it makes it less likely to be taken as an insult.  This is because the word modest has a generally positive connotation.  
See the definitions of modest at Merriam Webster:

1a : placing a moderate estimate on one's abilities or worth
  1b : neither bold nor self-assertive : tending toward diffidence
  2 : arising from or characteristic of a modest nature
  3 : observing the proprieties of dress and behavior : decent
  4a : limited in size, amount, or scope
  4b : unpretentious

The definition in use here is probably closest to 4a (which does use the word limited), but if you look at all of the other definitions, they are generally good things.