Usually, the - and + keys enter the characters - and +. However, when opening a document for viewing only, that can't happen. Perhaps we should consider accepting these keystrokes as zoom level changes. Playing with the zoom level is a very common action in read-only mode, and viewers typically support using - and + for this purpose.
Eyal, if you add keyword needsUXEval, please add also design team to cc list. Thank you. I've also changed component to LibreOffice, because I understand this a enhancement for every part of LO.
Using different shortcuts depending on some state makes it hard to familiarize. Might be not much of a problem in this particular case. But it also depends on the type of keyboard anyway (wouldn't press +/- on a TKL). Another argument against the modification is the required hard-coding. Last but not least I don't see need / a clear use case / some real life request for this. => WF