The inevitable conclusion of the day: Next time check for plagiarism with alternative tools before submitting your assignment, not after. There are tools which could detect similarities in different types of documents, including html and pdf - for example, Unplag. If you listen to my advice, you could prevent situations like that.
And don’t blame yourself - now you’re experienced enough and won’t cheat again. As Stephanee Killen has said, “Life is the best teacher, just as it is. It is the toughest teacher. It won’t tolerate slothfulness for long. It’s always throwing some difficult problem your way and then seeing what you will do with it.”