I send a PDF to my client and the colour 'red' appears 'black' on his computer

I send a PDF to my client through email. He says the colour 'red' appears 'black' on his computer. We are both Mac users. The 'red' I used in the document is a Pantone Colour. What is the problem? How do I fix this?

I opened the pdf in a Windows computer, the colour appeared 'red'.