Hello,
I didn't notice the forum this was posted in. For the cloud service, this should be handled by Alfresco.
In terms of your local repo: A txt is converted via the PDFBox library, not OpenOffice, so yes, it may very well be that fonts is not the main / only issue or not even an issue at all. For the odt, it could be an issue if the instance of LibreOffice used has different fonts than the OpenOffice instance on the Alfresco server - but I'd assume this as less likely than between MS Office and OpenOffice.
My next major suspect would be PDF2SWF. Are you able to test the PDF2SWF transformation by saving a copy of your odt from LibreOffice as PDF and manually invoking pdf2swf? This would help in verifying or eliminating that component based on the output.
Are you able to provide a sample odt file without sensitive information for a quick test?
Regards
Axel