Hello, can I ask for advice about embed files in Logseq mobile app (Android)? My files are stored in Dropbox and I need some of them to have a preview in Logseq. I found out that images and videos work but PDF and word/excel not. Could you please advise? Thank you.
Welcome to Logseq and the community @Lucie! 
Dropbox embeds with images and videos work because Logseq supports images and videos. On the other hand, Logseq does not support embedding Microsoft files like Word and Excel. PDFs currently only work with PDFs that you add to your graph.
You could experiment with using an iFrame on a whiteboard, or some simple Markdown or ClojureScript code. Here’s the documentation page about media embeds:
https://docs.logseq.com/#/page/embed%20media%20-%20audio%2C%20photos%2C%20videos