I'm going to add new content about Paypal campaign on my site. But its content is not that big enough for a page(50 -100 words). And I'm wondering if I should add it to the existing related page (e.g. the page for using fee) or make a new page just for the campaign.
Well, theoretically you could ask if it's better to split it into 200 pages of word each :)
I believe that "content is a king" should be your golden rule, so if you can split a big article into two different subjects and have enough content for each one of them to be considered valuable, then the answer is "yes".
Think about it this way, google tries to decide if your content is unique and has quality in it, this is more important than having a lot of pages. Further, if you have a lot of pages "at once" it is not considered as good as a website that is smaller but updates with new quality content more frequently. And since my answer will not be complete without some links, here are a few recommended articles