In case of you have added the reference to the SautinSoft.HtmlToRtf.dll by the old way (Solution Explorer->right click by "References"->Add Reference...), please add these dependencies using NuGet:

<PackageReference Include="System.IO.Packaging" Version="4.5.0" />
<PackageReference Include="System.Text.Encoding.CodePages" Version="4.5.0" />
<PackageReference Include="SkiaSharp" Version="2.88.7" />
<PackageReference Include="Svg.Skia" Version="1.0.0.18" />
<PackageReference Include="SkiaSharp.NativeAssets.Linux" Version="2.88.7" />
<PackageReference Include="SkiaSharp.NativeAssets.macOS" Version="2.88.7" />

For example, to add "System.IO.Packaging, 4.5.0:
(Solution Explorer->right click by "References"->Manage NuGet Packages...->In the tab "Browse" type "System.IO.Packaging" and find the "System.IO.Packaging", select version "4.5.0").

If you will use the dll on Windows platform only, you may remove SkiaSharp.NativeAssets.Linux and SkiaSharp.NativeAssets.macOS

Detailed steps:
Windows https://sautinsoft.com/products/html-to-rtf/help/net/getting-started/windows.php
Linux https://sautinsoft.com/products/html-to-rtf/help/net/getting-started/using-in-linux.php
MacOs https://sautinsoft.com/products/html-to-rtf/help/net/getting-started/mac-os.php

If you will have any questions, please don't hesitate to email us at: support@sautinsoft.com!