docs: document tmux active session bug (#402)

docs/src/guides/pdf-export.md

@@ -25,6 +25,13 @@ so make sure to adjust accordingly before running the command above.
 > Note: if you're using a separate virtual env to install _presenterm-export_ just make sure you activate it before 
 > running _presenterm_ with the `--export-pdf` parameter.
 
+### Active tmux sessions bug
+
+Because of a [bug in tmux <= 3.5a](https://github.com/tmux/tmux/issues/4268), exporting a PDF while having other tmux
+sessions running and attached will cause the size of the output PDF to match the size of those other sessions rather 
+than the size of the terminal you're running _presenterm_ in. The workaround is to only have one attached tmux session
+and to run the PDF export from that session.
+
 ### How it works
 
 The conversion into PDF format is pretty convoluted. If you'd like to learn more visit