encrypt/decrypt file by RSACryptoServiceProvider

I need to encrypt/decrypt files (type pdf,txt,doc) by using RSA algorithm in c# I import keys from XMl file

I use this method

public byte[] DecryptData(byte[] encrypted)
{
    int nBytes = encrypted.Length;
    byte[] ByteArray = new byte[nBytes];


    RSACryptoServiceProvider rsa=new RSACrypto开发者_如何转开发ServiceProvider();

    StreamReader reader = new StreamReader(@"E:\test\keyStore\Receiver\PrivateKey.xml");
    string PrivateKeyXML = reader.ReadToEnd();
    rsa.FromXmlString(PrivateKeyXML);
    reader.Close();

    //store decrypt data
    ByteArray = rsa.Decrypt(encrypted, false);
    ////convert bytes to string
    //ss = Global.enc.GetString(fromEncrypt);
    return ByteArray;

}

ERROR MESSAGE="The data to be decrypted exceeds the maximum for this modulus of 256 bytes."

please, help me

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This is because RSACryptoServiceProvider (assymmetric) should be used only to encrypt the symmetric key used for encrypt a document.