PDFs (Portable Document Format) are intended to be a secure way to distribute documents in a standard format that anyone can view. As long as the recipient has Adobe Acrobat Reader, they can view it. But generally, you would want your recipient to view the document without making changes to it. Otherwise, you would send them a Word document or some other easily editable text document. For most uses of PDFs, they can not be edited.
Yes, there are some PDFs that contain form fields that you can fill in. But the form fields are the only places you can type; you can’t make changes the document text. You can make a few minor edits or make annotations with Adobe’s Acrobat Professional, if you are willing to pay $200-$500 (upgrade vs. full version) for the program. But for the basic user, you cannot and should not edit a PDF.