.pdf is a format intended for making sure a document looks the same on all computers. It became popular because it allows the creator to make sure that the document looks the same and cannot be changed by viewers. These days it is an open standard and there is quite a variety of programs that can read and save pdfs.

Krita can open pdfs with multiple layers. There is currently no PDF export, nor is that planned. If you want to create a PDF with images from Krita, use Scribus.

While pdfs can be viewed via most browsers, they can also become very heavy and are thus not recommended outside of official documents. Printhouses will often accept pdf.