Excel Sync problem

[Paul_RobertsPaulie101]

Hi all, I'm a new user to this site, running a 2017 Mac pro with Monterey.

I just started having the following error when I try to open an Excel file. I receive the following ALERT-

"The file format and extension of '19 Schubert Hill NW Calgary, AB - Quote.pdf' don't match. The file could be corrupted or unsafe. Unless you trust its source, don't open it. Do you want to open it anyway? "... if I select no another file opens with the same message and then I'm in a loop. After I try to close the file I receive a Microsoft Error Report that says there was a problem and Excel will attempt to recover my work. When I click on the "More Information" button I see the error report but don't know what to do with it.  Attached are the screen shots ,of the error windows and details. Please help, Cheers, Paul

This is the window that pops up after I get out of the loopThis window pops up after I click No 3 times and quickle click the x to close the loop

[#2]

@Paul_RobertsPaulie101

The question you're being asked is "Do You Trust the File?"

Do you?

If so, go ahead and click on "Yes"

I get that same message with every file I download from my highly trustworthy financial services provider. For some reason, they make the Excel file that they export in a format where the file format and extension don't match. Do I trust them? Yes. So I open the file. And it works.

[#3]

Hello, Thanks for reaching out,

The first attachement I included is what pops up when I click Yes or No. When I click yes another random files tries to open and I get the same Alert message. @Paul_RobertsPaulie101 

[#4]

You may have accidentally put files into your Excel startup items folder. Remove them.