Because the use of paper documents is phasing out it has become more important to know how to work with computer files. The most common form of file to share information is the PDF document. For example, all Iowa DOT plans, specifications, manuals, addendum use the PDF format.
I think there are 3 main levels of working with PDF files View and Print, Create, Edit.
View & Print - Most computers come with a free pdf reader like Acrobat Reader to open files and print. Some of the forms that the Iowa DOT uses can only be opened using Acrobat Reader. If you try to open these files you will get this error.
Create - You need to be able to create your own PDF document. For example you could use Microsoft Excel to create your subcontract proposal but you should convert it to a pdf before you email it out. Why? The recipient may not have the same software as you and can't open it. Also a spreadsheet can be changed while a pdf is more permanent. You can create a PDF with Adobe Reader or PDFCreator
Edit - To be able to highlight, add notes, or mark-up a PDF you need a PDF editor. The original editor is Adobe Acrobat which costs about $600. I use Bluebeam Revu. It costs $350 plus it has functions like on-screen measurements and approval stamps.
For example you could markup a handicap ramp with dimensions, elevations and slopes from a table to create a plan that a crew could use in the field.