Mostly because it's the common format for documents where I live. Also you can add multiple images with different dimensions and they will all be scaled to fit A4. But I will probably add different dimensions for other countries in the future.

Being tied to specific paper sizes is one of the biggest limitations of PDF. I run into this a lot when people try to distribute documents as PDF. There are a lot of assumptions built in when a PDF is generated.

Agreed, I think the PDF should be created at the same size of the original image. Or at least have an option to do so.