A PDF file is a commonly used graphic file format. Many resources on TpT are in the PDF format. PDFs are best viewed with Adobe Acrobat Reader, which you can download for free from the Adobe website by clicking here.
Please note that in most cases, you won't be able to edit a PDF. If you prefer purchasing editable formats, look for resources that offer other file types (like Word or PowerPoint) instead. You can also specify "editable" in the search bar when you search the site for resources. Some PDFs are semi-editable, meaning that you will be able to add text boxes but not change the layout or background.