Running the pdf2htmlEX.exe Windows binary from the command prompt works as expected. While, running the pdf2htmlEX Windows binary in a wrapper (.Net in my case) I received an error like the one below.
__tmp_font1.ttf is not in a known format (or uses features of that format fontfo
rge does not support, or is so badly corrupted as to be unreadable)
Cannot load font C:\Users\admin\AppData\Local\Temp\pdf2htmlEX-5RLDCX/__tmp_fo
nt1.ttf
This is a pretty ambiguous error, and appears to be frequent among users when using the windows binary version.
Apparently Lu Wang wasn't able to offer a solution for Windows users, as all posts related are marked 'insufficient info'. Unfortunately, the pdf2htmlEX project is also archived, and no new comments can be added, so I'm adding this information here in the hope that this may help someone else in the future.
In my scenario, the library is called via an ASP.Net wrapper method using System.Diagnostics.Process to convert uploaded files into HTML versions. The Pdf2htmlEX library would work without issue from the Command Prompt, and for some reason, would also work perfectly in my development environment, but not in a production environment (Both of which are Windows Server 2012R2).
My first assumption, and correctly so, was that there was a permissions issue. Pdf2htmlEX uses FontForge internally to handle fonts, and one or both use the Windows Temp directory by default to store resource files used in the creation of the HTML and/or other files. And, I 'believe' although not confirmed, that it also may use the active user's %USERPROFILE%\AppData\Local\Temp folder...
When running test commands from Command Prompt, you are operating under your user context, and everything your user can do, Pdf2htmlEX can do. So everything works as expected.
In a server environment, the process is operating under the ApplicationPoolIdentity, a special IIS user type with limited permissions. Here it failed for me. While, I'd see folders and files created in the Windows Temp folder, they couldn't be opened by Pdf2HtmlEX to create the end files elsewhere.
Solution: (there may be other solutions for your individual case)
In my case, adding a new system user, adding that user to the Users group, and then setting the IIS worker process to that account resolved the issue. The reason I believe, is that the Users group has read/write access to the Windows Temp directory, and potentially other required areas of the system required for Pdf2htmlEX to complete.