Unable to read certain sections of a PDF on 2021 iMac running 13.1.

Hello barryfromgrandforks,

Thanks for choosing the Apple Support Communities. We see you're having a hard time opening an annotated PDF using Preview. Let's try following the steps in this user guide to see if they help resolve the issue.

Annotate a PDF in Preview on Mac - Apple Support

"Although you can’t edit the text of a PDF in Preview, you can use the markup tools to provide feedback or write down something you want to remember. The editing tools in the Markup toolbar are ideal for collaborating—use them to suggest changes to a PDF and to respond to suggestions.

1. In the Preview app  on your Mac, click the Show Markup Toolbar button  (if the Markup toolbar isn’t showing).
2. Use the tools in the toolbar to mark up the PDF (or use the Touch Bar).
3. Note: To save a PDF and allow annotations to be edited later, choose one of the following: File > Save, File > Export, or File > Export to PDF. (PDFs are also saved with editable annotations when they’re saved automatically.) To save a PDF and flatten annotations (so that they can’t be edited), choose File > Print, then select Save as PDF."

We hope this helps.

Cheers!