PDF files downloaded result in 0 byte files

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Hello: I have 3 large (between 15Mb and 25Mb) .PDF files stored on my C5 (current version) site. I found out that any attempts to Open or Save any of the files fails because C5 apparently sends a 0-byte file and calls the download finished.

Meaning, click on a link to said .PDF and choose "Open" (with Adobe Reader) fails. Do the same with choice "Save" starts & ends a download of the file name with file size 0.

I've tried re-uploading one of the .PDF files to confirm no corruption, but it behaves as the existing .PDF files which also will not download.

Suggestion?
Thx