using Microsoft.SharePoint.Client;

var username = "username";
var password = "pass";

var url = "https://myclient.sharepoint.com/";
var fileurl = "https://myclient.sharepoint.com/teams/folder1/folder%20x/somefile.docx";
using (ClientContext context = new ClientContext(url))
{
    SecureString passWord = new SecureString();
    foreach (char c in password.ToCharArray()) passWord.AppendChar(c);
    context.Credentials = new SharePointOnlineCredentials(username, passWord);

    Uri filename = new Uri(fileurl);
    string server = filename.AbsoluteUri.Replace(filename.AbsolutePath, "");
    string serverrelative = filename.AbsolutePath;

    Microsoft.SharePoint.Client.File file = context.Web.GetFileByServerRelativeUrl(serverrelative);
    context.Load(file);
    ClientResult<Stream> streamResult = file.OpenBinaryStream();
    context.ExecuteQuery();
    var file2 = streamResult.Value;
}