"The papers I refer to are mostly free PDFs from the internet on various topics" – then it's most likely one or both of:
- It is not a peer-reviewed paper published by a reputable journal. Be careful what you read, there are plenty of quack theories out there
- You are viewing a preprint, and by finding the article on the journal's website you will find the date. Also, the final version of the article will probably be better formatted, and may be better written and have mistakes corrected (this depends upon the stage of publication at which the preprint was circulated).
Many preprints are circulated prior to publication, so you may not find a better version yet. It's still really poor form for an author not to date a preprint. If a preprint is a few years old and has not yet been published, you have to wonder why.
I can not recall ever seeing a journal that does not put the journal name, date, and issue number on at least the first page. It would be very poor practice not to do this, because taken on its own it is impossible (without further research) to tell where the article came from. It's common even to date every page with the journal name, date, and issue number.
I suspect that if you're seeing this trend in most of the "papers" you read, that you really aren't reading research articles written by legitimate researchers. Ask your advisor or lecturer for the names of the most relevant journals in the field, and start from there.