The process cannot access the file because it is

Could you please help me out? I know it's my own process (encrypt) but have no idea how to solve it and delete the file. it throws error when hits file.delete Here is my code and error.

//Encrypt file and save in the download directory
Encrypt(Server.MapPath("~/SecPages/Documents/DocTmp/" + randomFileName + PostfileExt), Server.MapPath("~/SecPages/Documents/Downloads/" + PostfileName + PostfileExt));

//Delete file
File.Delete(Server.MapPath("~/SecPages/Documents/DocTmp/" + randomFileName + PostfileExt));

private void Encrypt(string inputFilePath, string outputfilePath)
        {
            string EncryptionKey = "MAKV2SPBNI99212";
            using (Aes encryptor = Aes.Create())
            {
                Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });
                encryptor.Key = pdb.GetBytes(32);
                encryptor.IV = pdb.GetBytes(16);
                using (FileStream fs = new FileStream(outputfilePath, FileMode.Create))
                {
                    using (CryptoStream cs = new CryptoStream(fs, encryptor.CreateEncryptor(), CryptoStreamMode.Write))
                    {
                        FileStream fsIn = new FileStream(inputFilePath, FileMode.Open);
                        int data;
                        while ((data = fsIn.ReadByte()) != -1)
                        {
                            cs.WriteByte((byte)data);
                        }
                    }

}
}

}

Error

The process cannot access the file 'K:\87.pdf' because it is being used by another process.

{

string EncryptionKey = "MAKV2SPBNI99212";

using (Aes encryptor = Aes.Create())

{

Rfc2898DeriveBytes pdb = new Rfc2898DeriveBytes(EncryptionKey, new byte[] { 0x49, 0x76, 0x61, 0x6e, 0x20, 0x4d, 0x65, 0x64, 0x76, 0x65, 0x64, 0x65, 0x76 });

encryptor.Key = pdb.GetBytes(32);

encryptor.IV = pdb.GetBytes(16);

using (FileStream fs = new FileStream(outputfilePath, FileMode.Create))

{

using (CryptoStream cs = new CryptoStream(fs, encryptor.CreateEncryptor(), CryptoStreamMode.Write))

{

FileStream fsIn = new FileStream(inputFilePath, FileMode.Open);

int data;

while ((data = fsIn.ReadByte()) != -1)

{

cs.WriteByte((byte)data);

}