PDF files damaged after they were turned to zip files

[vforl1]

 Some days ago I created two new files in my dropbox and I moved some pdf archives in them from a third file. Some seconds later I turned these two files to zip files using windows zip function (all these steps in my desktop dropbox). No message appeared that something went wrong.
Unfortunaterly  I found out later that all of the pdf archives can not be opened. The ACROBAT message is "......the file has been damaged"

What happened?

Is there any change  in dropbox application and my dropbox files are mirror of the cloud and not saved in hard disklike in the past? (Because i noticed that the icons changed.)

[Hannah]

Hey @vforl1, thanks for reaching out to the Dropbox Community.

Dropbox doesn't really alter your files in any way, when you upload them either through the website or the desktop app, but let's see what we can find.

Ar the files that are giving you the error set to "online-only" or a"available offline"?

If they're currently online-only, please right-click on them and make them available offline, to see if the issue persists.

Let me know what you find.