how to hide text in a PDF in Preview
Open it with a PDF editor, like Acrobat Standard, and actually remove the text you want gone, then save it.
If you don't need the text to be selectable or searchable you can try this trick:
After blocking out the text you want hidden, export the document to TIFF format. You do this via the menu item File > Export... and then choosing TIFF from the format menu at the bottom. This will create a large image file.
Open this file and export again but this time back to PDF.
You'll end up with a PDF containing an image of each page instead of text.