Check pdf using C# Code

public bool IsPDFHeader(string fileName)
    {
        byte[] buffer = null;
        FileStream fs = new FileStream(fileName, FileMode.Open, FileAccess.Read);
        BinaryReader br = new BinaryReader(fs);
        long numBytes = new FileInfo(fileName).Length;
        //buffer = br.ReadBytes((int)numBytes);
        buffer = br.ReadBytes(5);

        var enc = new ASCIIEncoding();
        var header = enc.GetString(buffer);

        //%PDF−1.0
        // If you are loading it into a long, this is (0x04034b50).
        if (buffer[0] == 0x25 && buffer[1] == 0x50
            && buffer[2] == 0x44 && buffer[3] == 0x46)
        {
            return header.StartsWith("%PDF-");
        }
        return false;

    }