As I known, for Azure WebApp on Windows, any app using GDI is not supported, because there is a Win32k.sys (User32/GDI32) Restrictions introduced by the offical wiki page
Azure Web App sandbox, as the figure below.

However, wkhtmltopdf
without using GDI is a special framework which can be used on Azure WebApp for Windows, but you have to install it first on Azure WebApp without GUI interactive.
So the only way is first to download the MXE (MinGW-w64)
release files of 7z
archive format for Windows from the downloads page of offical site or GitHub releases page, as the figures below,
Fig 1. The offical downloads page

Fig 2. The GitHub releases page

And to unzip it on local and add it into your PHP project, then append the bin
path of wkhtmltopdf
to the tail of PATH
environment variable via PHP functions getenv
and putenv
at the start of your PHP app or try to set the PATH
value via add a key PATH
in appSettings
of web.config
file.
Finally, you can use the 3rd party library mikehaertl/phpwkhtmltopdf
to do the convert function from HTML to PDF on Azure WebApp for Windows.