A family of Microsoft word processing software products for creating web, email, and print documents.
On my PC, I have no trouble pasting multiple lines or multiple paragraphs into a plain text content control set for "allow carriage returns". So it isn't a limitation or bug in Word 2013.
I would focus on two possibilities.
- What kind of text in the old documents are they copying from? Is it paragraphs of regular text and/or bullet lists, the contents of protected form fields, text within tables, or other special circumstances? (Although I've tried all of those, and they just work...)
- Is there an add-in -- common to all the PCs that have the problem -- that may be interfering? Try restarting Word while holding down the Ctrl key; when it asks whether to start in Safe Mode, answer Yes, and then try copying and pasting the same kind of text. Safe Mode prevents the loading of any add-ins, as well as the Normal.dotm template and any special settings in the registry. If the problem stops then, try disabling the add-ins one at a time to see whether you can isolate one that's causing the problem.