Hi Richard,
I would bet the Mac you're creating these PDFs from is not embedding the Helvetica font. So when you open the resulting PDF, the Mac doesn't know what font is supposed to be used for the text, and displays everything with LastResort, which will give you a bunch of boxes. Or, the installed Helvetica font is damaged in some way. Not enough that it won't display correctly in Pages, but enough that the embedded copies get trashed.
I didn't see a way to check a PDF in Preview to see what fonts are part of a PDF. Doesn't seem to have that option. So if you would, please install the free Adobe Acrobat Reader. Open a non-functioning PDF with it and press Command+D. Click on the Fonts tab. It should show the embedded fonts, such as this partial list for a PDF from Apple for a 2008 Mac Pro manual.

If it shows nothing, then my guess is correct. Helvetica isn't even being embedded. If it does show Helvetica as being part of the PDF, then they're somehow damaged. You can replace Helvetica with a fresh copy by extracting it from the OS X installer package, using Pacifist. Or just grab a known good copy from another Mac of the same version of OS X/macOS and replace the one currently on the drive.