Hi Sergei Shibanov,
PFB my answer of your queries.
Is it possible use only first two lines for reproduce issue?
Answer- Yes
Answer - htmlString as read html from file as string and pass it, which I have shared.
Htm…...HtmlLoadOptions()); // save document in PPTX format document.Save(“output.pptx”...HtmlLoadOptions()); // save document in PPTX format document.Save(“output.pptx”...