PDF stands for Portable Document Format. It was created so a document looks identical everywhere — the same fonts, images and layout whether you open it on a phone, a laptop or a printer.
That reliability is why contracts, invoices, forms and ebooks are usually shared as PDFs. The trade-off is that PDFs aren't meant to be edited as easily as a word processor document.
To work with PDFs you can merge, split, compress, convert or sign them — all free and in your browser.