Won't let me open email attachments

For the first time today I was unable to open an attachment from an email that was sent to me, and I got the following error message. I've never had this issue before, and this is a trusted email that I regularly receive emails with attachments from. I was able to open an attachment from them even a few days ago. When this happened, I tried to open other attachments from other emails and got the same error message. 
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I'm using AVG free.

Hi Brittany,

We're glad to look into this & help you.
From the screenshot, it seems that you're accessing emails via Chrome & receive this message upon attempting to open a pdf attachment.
Would you be so kind to clarify it & if so, try accessing the emails in different browser or email clients to let us know the outcome?
Also, let us know whether you use AVG Free Antivirus on your computer along AVG Secure VPN.

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I tried opening the email attachments in Firefox and Microsoft Edge and it still wouldn't let me open them in either.

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Thank you for the screenshot, Brittany.
We should check which component of AVG is detecting this threat.
It should be email shield and to confirm this, we request you to send the screenshot of entire threat detection popup.

Brittany, could you share the screenshot of the error that you received for accessing emails?
You can post the screenshot here in your topic. Click on 'Answer' & then click on the 'Image' [mountain symbol] & follow the instructions.