PDF stands for Portable Document Format. These are the types of documents you get when you download files from web sites, textbook manufacturers, and other online sources. They are meant to be viewed or printed in very specific formats. If you get digital worksheets from a textbook or from Teachers Pay Teachers, they are often in pdf format. They are generally not meant to be edited. For example, many pdf documents are meant to be printed on an 8.5″ x 11″ sheet of paper. They are formatted to barely fit on an sheet of paper and any editing would mess up their printing. That is why they are often not editable. Yes, they can be made editable (like tax documents), but it is a pain to set them up that way or change them to be editable.
Most text based documents (Word, GDocs, Slides, PowerPoint, web pages, etc.) are easy to convert to non-editable pdf documents. Making editable pdf documents, though, is a pain and very time consuming. Trying to convert pdf documents to Word so that they can be edited is also a pain. If you want students to edit a pdf document, it may be faster to retype the questions in Docs or Slides than to try to make the pdf editable.
Access for Students and Parents
They are universally readable by most Internet connected devices. They are just not easily editable.
Updating and Modifying
I generally don’t bother with it because it takes too long. I can recreate things faster in Docs than make the pdf editable. There are free and pay tools for making pdf’s editable, but it makes things more complicated, especially if you now want the students to be able to edit them.
They have been around for a while and will be too. Any pdf files you have will be good for a long time.
Reading pdf files requires special software, but it is free and often installed on most devices nowadays. Acrobat Reader is the most common pdf reading software and is free. Creating good, editable pdf files requires software that may cost money (like Adobe Acrobat).
Very limited. I use them when I want to print documents that I got from someone else, like a website or the textbook manufacturer. I only create them when I want people to be able to print or view a file with specific formatting and not be able to edit it. My class syllabuses are posted to my website as pdf documents. I don’t deal with students trying to modify pdf documents.