Problem with attachments in Mail

Hi. I am having trouble downloading PDF attachments in Apple Mail. I've only noticed the problem with PDFs. I've had no problems with Word docs.

Here's what's happening:

When someone sends me a PDF attachment, it shows the word "Downloading..." under the PDF icon within the email. When I click on the icon, or the paperclip button in Mail, to save the PDF to my hard drive, it doesn't work. Nothing happens. And the word "Downloading" does not go away over time.

If I log into Gmail via my web browser, and access the email that way, I can download the attachment.

I only noticed this issue in the last few days. And it's happened in emails from two different people.

I upgraded from Sierra to High Sierra last week so maybe that's a factor.

If anyone can explain to me what might be going on, and how to fix the issue, I would be grateful.

Thanks!

M