Libre office Version: 6.2.0.3 for Mac (OS Mojave version 10.14.3 (18D109))
I created a text document with comments and saved it as docx. When I export the document to PDF (I use 'Export as PDF) and select export comments in the General Tab, the comments do not appear in the PDF. I open the PDF with Preview (pdf-reader for Mac). I see only little white squares. When I click on them the comments are empty.
How can I resolve this problem?
6.1.2.1 is quite old in the 6.1 branch: current version is 6.1.5.2. In the commit logs, I see a number of commits pushed after 6.1.2, related to comments. First thing to do in case of a bug is to check if it’s fixed in a later release 
Thank you for your answer. I thought I had the latest version installed. I will install the latest version. 
I installed the latest version 6.2.0.3 and the problem still remains.
I cannot reproduce using 6.2.2.1 on Win10. So - could you please provide both a sample docx with such problem, and a PDF, so that we could see if the problem is specific to some documents or some PDF viewers. (By the way, what if you open the PDF using LO itself? Would Draw show the comments?)
I found a workaround solution. I used ‘Print’/‘More’/‘comments place in margin’/‘save as pdf’. That way the comments are visible.
When I open the pdf created through Export as PDF in Draw, the comments do not show.
It is not allowed to upload pdf-files.
Tested in LibreOffice 7.1.3.2:
Do NOT go to File → Export As → Export as PDF…
Instead, go to File → Print → under the “General” tab, choose “Print to File…” as the Printer → click the “LibreOffice Writer” tab at the top → under the “Comments” dropdown menu, change it from “None (document only)” to “Place in margins” → click “Print to File…” and choose a location to save it.
Big thanks to @Izabel for pointing this out in the comments of the question!
See also my other, related answer here: How to print a Google Docs document with comments in the margin? - Web Applications Stack Exchange.