The user I'm sharing a folder with can't open PDFs on their end.

No, these settings wouldn't be affecting your files and how you preview them Colene 

Have you ensured that your colleagues are trying to open those files after making them available offline on their computers? 

Alternatively, could any of the below settings be an issue?

Hi Rich, 

Do you perhaps have a solution for an error like this:

I've suggested all of the above to my colleague but they still can't access some of the files

This is very helpful - I will go check up on this as well

Thanks Walter, I'll check with them in the morning

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Colene wrote:

I believe it's offline, my collaborator doesn't work online. 

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Just to be clear, this isn't referring to how someone works with their Dropbox, but how the files are marked in their Dropbox folder. If you've added files to a shared folder, they could be displayed initially on their end as Online-only, meaning the files are only in the cloud and a placeholder is displayed on the local computer.

Some application have a problem opening files that are marked as Online-only. Basically, they try to open the file and don't wait long enough for it to download, resulting in an error about the file being missing, the wrong type or corrupt.

Ask the other person to right-click one of the files and select Make available offline, wait for the file to sync and display a solid green check mark, then try to open it.

In that case, can you ask them to rename it or move it to another location and see if they can open it from there Colene?

I believe it's offline, my collaborator doesn't work online. 

Thanks for the screenshots @Colene 

Can you check with your collaborator(s) if they get this error when the file is online-only or even after making it available offline on their computer(s)?