Yes, it is okay.
But I don't undestand why you want to make it more difficult for potential clients to read your work.
If you wrote something that you have permission to use in your portfolio, go ahead and include the actual article in you portfolio.
re: "You mean just upload individual articles?"
Yes. That's what the portfolio is for.
Upload individual articles you have written. As long as you have permission to do so.
And often, if there is any question, you can ask the owner of the article (if it isn't you) if you may have permission to include it in your portfolio.
Remember that portfolio items may contain links to other websites.
If possible, include the actual article in your portfolio, and a link to where it was published online.