When I copy a paragraph from a PDF and paste it into Word I get random symbols instead of letters. The PDF looks fine on screen. Is it broken?

The file is not broken. The cause is how the fonts were included.

A PDF stores each letter as a glyph number. A normal font has a map that says "this glyph is the letter A". When the map is missing or custom, the PDF displays correctly but copying gives garbage.

Options:

1. Run OCR on the PDF. Acrobat's Recognize Text can read the page as a picture and create a clean text layer.
2. Ask the creator to export again with fonts embedded and a standard font encoding.
3. Print the PDF to a new PDF and then run OCR on that.

If you made the PDF yourself, use Save As PDF from the original program and avoid unusual or very old fonts. Check File > Properties > Fonts in Acrobat to see what the file uses.

OCR gave me clean text. Thanks for the explanation.