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Using the example URL -- http://www.hungrymindlab.com/wp-content/uploads/2015/10/Larsson-and-von-Stumm-2014.pdf -- in your question, we can view the article and see at the top of the PDF all the information about the author, title, journal, etc.

There are several ways we can try to get a bibliographic reference for this article.

1)  We could use the Google Scholar button while viewing the PDF.  See a step-by-step tutorial here:

 http://ior.ad/hN1

2)   We could use Google Scholar -- http://scholar.google.com -- and search for the article.  See a step-by-step tutorial here:

 http://ior.ad/hN3

3)   We could use a tool like Citefast.com and choose to add a "Journal article" and let the tool search for the article by title and/or author.  See a step-by-step tutorial here:

 http://ior.ad/hN8

Here is an APA citation for this article:

Larsson, E. E., & Von Stumm, S. (2015). Seeing red? The effect of colour on intelligence test performance. Intelligence, 48, 133-136.

Note that the IBO does want a link included, if possible.  So you can add that information to the end of the citation.

Larsson, E. E., & Von Stumm, S. (2015). Seeing red? The effect of colour on intelligence test performance. Intelligence, 48, 133-136.  Retrieved from http://www.hungrymindlab.com/wp-content/uploads/2015/10/Larsson-and-von-Stumm-2014.pdf.

Content that is unlikely to change (such as articles published in journals) do not need a retrieval date.  If information is retrieved from a general website, then a retrieval date should be included, e.g.,  "Retrieved May 25, 2017, from http://www.blah.com/generalinfo."

The in-text citation in APA would be:  "blah blah blah (Larsson & Von Stumm, 2015)."