First, read the documentation on sending files.

Uploading the actual file

using (FileStream fs = System.IO.File.OpenRead("Local file.pdf"))
{
    InputOnlineFile inputOnlineFile = new InputOnlineFile(fs, "Name for the user.pdf");
    await Bot.SendDocumentAsync(message.Chat, inputOnlineFile);
}

Uploading by file id

This is the method that you will want to use the most, as it is the most efficient. After sending a file to the user, or receiving a file, you can send the file again using its ID.

This saves you from uploading the entire file.

InputOnlineFile inputOnlineFile = new InputOnlineFile("file id");
await Bot.SendDocumentAsync(message.Chat, inputOnlineFile);

Uploading by URL

InputOnlineFile inputOnlineFile = new InputOnlineFile("telegram.org/img/t_logo.png");
await Bot.SendDocumentAsync(message.Chat, inputOnlineFile);