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You can use the .NET Framework 4.8's System.Security.Cryptography namespace to generate PEM certificates.

Here's an example code snippet that generates a self-signed certificate and exports it in PEM format:

using System;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Text;

public class Program
{
    public static void Main(string[] args)
    {
        // Generate a new RSA key pair
        RSACryptoServiceProvider keyPair = new RSACryptoServiceProvider(2048);

        // Create a self-signed X.509 certificate
        X509Certificate2 certificate = new X509Certificate2(
            keyPair.Export(X509ContentType.Cert),
            null,
            X509KeyStorageFlags.Exportable);

        // Export the private key in PEM format
        string privateKeyPem = ExportPrivateKey(keyPair);

        // Export the certificate in PEM format
        string certificatePem = ExportCertificate(certificate);

        Console.WriteLine("Private key (PEM):\n" + privateKeyPem);
        Console.WriteLine("Certificate (PEM):\n" + certificatePem);
    }

    public static string ExportPrivateKey(RSA privateKey)
    {
        byte[] privateKeyBlob = privateKey.ExportRSAPrivateKey();
        StringBuilder builder = new StringBuilder();
        builder.AppendLine("-----BEGIN PRIVATE KEY-----");
        builder.AppendLine(Convert.ToBase64String(privateKeyBlob, Base64FormattingOptions.InsertLineBreaks));
        builder.AppendLine("-----END PRIVATE KEY-----");
        return builder.ToString();
    }

    public static string ExportCertificate(X509Certificate2 certificate)
    {
        StringBuilder builder = new StringBuilder();
        builder.AppendLine("-----BEGIN CERTIFICATE-----");
        builder.AppendLine(Convert.ToBase64String(certificate.Export(X509ContentType.Cert), Base64FormattingOptions.InsertLineBreaks));
        builder.AppendLine("-----END CERTIFICATE-----");
        return builder.ToString();
    }
}

Note that this example exports the private key in unencrypted PEM format, which is not secure. If you need to export the private key in encrypted PEM format, you should use a more secure implementation of the ExportPrivateKey() method.