How to achieve below one : merging documents

@kkishore, assuming that the last object will be in the format that you provided, you can implement the following solution.

1. Group by N - to group the input data in a single array
   
2. Mapper
   Use the following expression, with this you can add the 'count' object to each input document.

$group.filter(val => val.get("count") == null).map(val => val.extend($group[$group.length - 1]))

Target path: $array

Result:

{
    "array": [
      {
        "filename": "a1.dat",
        "count": 2,
        "type": "pdf"
      },
      {
        "filename": "a2.dat",
        "count": 2,
        "type": "pdf"
      }
    ]
  }

After this, you can use a JSON Splitter if you want to split the array.