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);