Daniela:
Keep in mind that the purpose of a portfolio is: advertising.
If you want files to be "secret" or "un-copyable" or completely "protected", then don't put them on the Internet. Anywhere.
Watermarking your files with something that identifies them as having been created by you is a great idea. I don't think anything more needs to be done for portfolio items.
The kind of "people" (and I'm using that word to be polite) who copy portfolio items from other people are generally going to be too lazy to fiddle around with your portfolio pieces if you do that.
But technically speaking: no matter what you do, if you put something on the Web, a truly determined person can remove watermarks or "steal" your work.
If you actually look closely, you'll see that the overwhelming majority of portfolios on Upwork do not use watermarking.
