I already asked that exact question over at Superuser.com and was referred here in a comment. The question is reproduced below the horizontal rule.
The document in question can be downloaded from here.
On Windows, using LibreOffice 18.104.22.168, when I create a presentation using Impress, the Slide Show (F5) cuts off the bottom and top (similar to wide-screen movies) and thus changes the aspect ratio, messing up the fonts. What gets messed up in particular is the spacing between letters, which gets stretched a lot.
Here's how it looks during creation:
Here's how it looks on the screen with the preview (fine, too):
And now the messed up presentation as it appears on my screen (which has the same aspect ratio as our projector, 4:3):
I also noticed that the framing with the background you choose during initial creation of the presentation is not even shown on the last view.
How can I make sure that the aspect ratio (and consequently the inter-letter spacing and so on) remains the same, no matter what?