Thank you for the massive response, which says a lot about SDL and this community. 

Maria Gracia Navas Quintana

Which filetype in Studio is being used for these files?  You can see this in the files view in the "File Type Identifier" column.  Do you know what version of Studio was used to prepare the package?  Have you tried sending the files back to whoever created the package for you to see if they can save the target and open them without a problem?

Hi Maria,

I too have had exactly that same problem several times. However, it only happens with PowerPoint files from one customer and not from any others (at least not so far). The files are not sent to me in a package and have not been preprocessed in any way in Studio. I get round the problem by sending the original and the Studio file to a colleague and asking him to save as target for me. That always works fine, so there's presumably some little setting in my Studio version that needs tweaking. Unfortunately, I have yet to find out what.

Martin