In PubMed, filter your results by using the options on the left sidebar. Click on one or more filters to include those attributes. There is no filter for PRIMARY articles but you can filter your results by article type.
ARTICLE TYPES: Try Clinical Trial, Randomized Controlled Trial, or another type of study appropriate for your needs. If you do not see the type of article you need click on the Customize link to see more options and add them as a category.