You shouldn't be using the BinaryFormatter for this - that's for serializing .Net types to a binary file so they can be read back again as .Net types.

If it's stored in the database, hopefully, as a varbinary - then all you need to do is get the byte array from that (that will depend on your data access technology - EF and Linq to Sql, for example, will create a mapping that makes it trivial to get a byte array) and then write it to the file as you do in your last line of code.

With any luck - I'm hoping that fileContent here is the byte array? In which case you can just do

System.IO.File.WriteAllBytes("hello.pdf", fileContent);

The string from the response seem to be Base-64 encoded (ref. fiddle). You can decode it using fetch() (or XMLHttpRequest() for older browsers):

fetch("data:application/pdf;base64," + response.data)
  .then(function(resp) {return resp.blob()})
  .then(function(blob) {
    FileSaver.saveAs(blob, 'foo.pdf')
  });