Originally because I didn't find a free app which would do that. Then I decided to keep the PDF as it was quite useful. Unlike with the PNG, the HTML links were retained in the PDF. Ie, HTML anchor tags are still clickable in PDFs generated by wkhtmltopdf.

EDIT: A PDF will also let you select text, unlike an image. However, an image is nicer to embed in a webpage. So I utilised both in order to get the best of both worlds.