The problem I see in this thread is that it's not clear what you want. It's especially not clear because what I think you probably want based upon your first post is not what you have subsequently tried to generate.
Your original post says "large size file with some random characters and numbers". Some people may consider a number to be a character which would make your specification of numbers redundant.
I would guess you're looking for a file that contains something like
k'povjyhk 'onu*y'y*Y*@%6nb4u;obu@*^@y4'@u@oI@POU^'9u49N@(y54'8y@VVY@£8yB4'Y'm'mopi'Ejoi'hgo'\smhoh'OHdohedo'hOI@HDFoDH@M C'u*$£@coiOINnhoiNY;oby;oby;oiYHouyhouyn 8y979^uighkjg86t*gn y *(ny(Pnt789Tb&* yy'89'Y'8ny&*(
That's the result of my randomly hitting the keyboard whilst trying to hold down a shift key some of the time. one way to generate a lot of that would be to use one of the methods you can easily find only for generating random passwords. Do it in a loop until you have what you consider to be sufficient data.
The first reply you got suggested two methods, one of which would result in a bunch of pseudo random numbers and the second, using dd, which would generate binary data I suspect you don't want binary data. You have chosen the dd command and now you seem to be complaining that it hasn't produced what you want.
I suggest that you post an example of the sort of content you're trying to get and also explain what you mean by 'large file'. (Are you thinking in terms of KB or MB or GB, or hundreds of characters or thousands of characters or tends of thousands of characters, or what?)