If it's a journal article in a reputable journal, you should contact the editor, who will then investigate. If plagiarism is detected, they will retract the article and inform the competent authorities in the offending researcher's university. It is a matter of professional courtesy to also alert the person whose work has been plagiarised.
I agree with the above but posts in other threads in RG suggest that certain journals do not take action. In such a case it seems there is little to be done.
Plagiarism can be about either the ideas, or the expression. For the idea it is only a matter of scholarly ethics, that isn't protected by the law. The publisher or the university should be contacted. An arrangement with the plagiarist can also be sought.
If a writing has been copied, or even paraphrased, that is, the expression, the form, the way of exposing things is the same, it is a legal case. This is protected by the Berne convention, the last version of which is the Paris act and the WIPO Copyright Treaty. There is a procedure available in every signatory country.