How to Add a Text Box to a PDF on iPad
Download Highlights for free. Not on an iPad? Checkout the Mac and iPhone guides.
1. Open the PDF document in Highlights
Locate and tap the PDF file you want to open in the Highlights document browser:

The PDF opens in a document scene:

2. Select the text or element where you want the text box
If you use an Apple Pencil or mouse, this will bring up the full selection popover:

If you use your finger, this will bring up the standard selection popover (without color options).
3. Tap the text box button in the selection popover

The text box note appears at the selected location and an annotation popover appears:

4. Type your comment in the popover

Click outside the popover to close it. Your comment is added to the text box and the text shows up in your notes which you can easily share.
You can also add comments to other types of PDF annotations including highlighted text and image selections by clicking on them and typing in the text field of the annotation popover.