Hmm, I wonder why Firefox thinks it's a PDF.

What kind of file is it?

Is the server sending a full file name such as whatever.docx or duck.jpg, or is the server omitting the file extension and letting browsers improvise?

Note: It might be helpful to set Windows not to hide any file extensions while sorting out this problem: https://www.bleepingcomputer.com/tutorials/how-to-show-file-extensions-in-windows/

Normally unless a web server sends a Content-Type header indicating what follows is application/pdf then Firefox wouldn't assume it's a PDF. It's possible that Firefox has stored additional associations. To investigate what Firefox has stored, you could try this:

Open your current Firefox settings (AKA Firefox profile) folder using either

• "3-bar" menu button > "?" Help > Troubleshooting Information
• (menu bar) Help > Troubleshooting Information
• type or paste about:support in the address bar and press Enter

In the first table on the page, on the Profile Folder row, click the "Open Folder" button. This should launch a new window listing various files and folders in File Explorer.

Resize/position the window so you can see the Troubleshooting Information page behind it.

Find the handlers.json file and drag and drop it onto the Troubleshooting Information page to open it in that tab.

After a few moments, Firefox should display a structured view of the data in that file. Use Find (Ctrl+F) to search for pdf and see whether it shows up under any other entries beyond the one for application/pdf that is normally included.

If the handlers.json file has bad data in it, because it is difficult to edit, we usually suggest renaming it to handlersOLD.json -- while Firefox is closed -- and letting Firefox create a new one at the next startup.

It's a .dmg file.

The server provides the entire file name that ends in .dmg

I'll try what you suggest. Thank you.

A .dmg is an executable file, so Firefox definitely should not be adding any kind of document extension (.pdf, .jpg, etc.) to the name. (!!)

Note that a .dmg file is a Disk Image file for Mac and it wouldn't make sense to download such a file on Windows (you posted with a Windows user agent).

Yes. I'm trying to get the dmg file intact (don't want to open) so that I can zip and then load to google drive for my Mac user students. But I don't know why it downloads as a pdf. Is there something I did?

What do you mean when you say it downloads as a PDF. Is the filename amended to have a .pdf at the end? Or does Windows think a .dmg file is something that should be opened in Adobe Acrobat/Reader?

Here’s what I had to do get around this issue: 1) I went to windows and change the program associated with .dmg files.

       a.	I changed the program associated with dmg files from PDF   (don’t know how that got there) to WinZip

2) After I downloaded dmg file in Firefox I clicked “Save As” and selected the WinZip program. 3) I then unzipped it within the program 4) I’ve now posted to my Google doc the “dmg.app” file. 5) I’ve asked a MAC user student to see if he can download it.

Good news. I don't know how Windows got confused about .dmg files in the first place, but your new association makes a lot more sense. You still might want to post the original .dmg file in case the Mac prefers it that way.

Thank you.