How can I convert a PDF to a Numbers document?

PDFs come in many different flavors. Most of them don't directly translate into a spreadsheet format.

Indeed, Numbers cannot open a PDF file directly.

IF the PDF contains tabular data, AND if it's saved in a way that identifies the table, then you MAY be able to open the PDF in Preview.app and select the table (click and drag over the text to select it). If all that works you MIGHT be able to paste that into Numbers and get something approximating the table from the PDF.

There are many gotchas to this, though, depending on the source of the PDF and how it's been saved. You might find you get the entire table, you might or might not get formatted text (colors, styles, backgrounds, etc.). You might just get a blob of text. Without trying it there's no way to know for sure.