It is pretty easy to add shapes in Adobe Acrobat DC so long as you know the steps. You can quickly insert a rectangle in a PDF with this highly-rated PDF annotator. Here is the way to do it:
Step 1: Open your PDF in this application;
Step 2: Choose Comment from the right column and tap the drop-down arrow next to Drawing Tools;
Step 3: Select Rectangle. Then drag your cursor to draw a rectangle.
This PDF program is good at adding shapes to PDF files to highlight the text. You can try to draw lines or add other shapes to your PDF.