It is said this way, and you may hear it in speech, but in written English you will not see it as its not technically the correct usage. Laurence stated an example that is much better.

A better sentence would be: I came across an important document while looking through a file to be thrown out.

Hi Niwantha, We "ran into" someone when we met someone by chance. Of course, we can also use "ran into" (something) when someone knocked into or collided with it. Example: He was talking to someone behind when he ran into a lamppost. It sounds odd that you "run into" an important document. I would suggest "came across". We "came across" something while seeing or reading it accidentally. We can also use "come across" for a person. Reference URL: https://idioms.thefreedictionary.com/came+across (look under definition 3). I hope this helps.