docfx: PDF Build fails in Azure DevOps

Operation System: Windows

DocFX Version Used: DocFx Build Tasks - Visual Studio Marketplace Extension

Template used: pdf.default

Additional Information: I am able to create the PDF locally just not in a build pipeline in Azure DevOps.

Steps to Reproduce:

1. Get Sources from repository
2. Add Chocolatey task to Install wkhtmltopdf
3. Run Create DocFx task to create PDF
4. Project File “docfx_pdf.json” <-- Just testing PDF build. Content builds work correctly.

"pdf": {
        "content": [
            {
                "files": [
                    "**/*.md",
                    "**/**.yml"
                ],
                "exclude": [
                    "**/toc.yml",
                    "**/toc.md"
                ]
            },
            {
                "files": "pdf/toc.yml"
            }
        ],
        "resource": [
            {
                "files": [
                    "**/*.png",
                    "**/*.jpg",
                    "**/*.gif",
                    "**/*.pdf",
                    "**/*.pptx",
                    "favicon.ico"
                ],
                "exclude": [
                    "**/obj/**",
                    "**/includes/**",
                    "**/_site_pdf/**"
                ]
            }
        ],
        "overwrite": [],
        "dest": "_site_pdf"
    }

Expected Behavior: Create PDF file

Actual Behavior: PDF file is not create with these errors in the Build Pipeline.

[19-05-06 06:59:08.197]Error:[PdfCommand.PDF]Error happen when converting pdf/toc.json to Pdf. Details: System.InvalidOperationException: The file is not a valid PDF document.
   at PdfSharp.Pdf.IO.PdfReader.Open(Stream stream, String password, PdfDocumentOpenMode openmode, PdfPasswordProvider passwordProvider)
   at Microsoft.DocAsCode.HtmlToPdf.HtmlToPdfConverter.SaveCore(Stream stream)
   at Microsoft.DocAsCode.HtmlToPdf.HtmlToPdfConverter.Save(String outputFileName)
   at Microsoft.DocAsCode.HtmlToPdf.ConvertWrapper.<>c__DisplayClass7_0.<ConvertCore>b__1(ManifestItem tocFile)
	24 Warning(s)
	1 Error(s)
##[error]Unable to run command: d:\a\1\s\docfx_pdf.json
code: 1

[19-05-06 06:58:57.112]Info:[PdfCommand.BuildCore.Build Document.LinkPhaseHandlerWithIncremental.Apply Templates]Applying templates to 1029 model(s)...
[19-05-06 06:58:57.707]Info:[PdfCommand.BuildCore.Build Document]XRef map exported.
[19-05-06 06:58:58.568]Info:[PdfCommand.Postprocess]Manifest file saved to manifest.json.
[19-05-06 06:58:58.661]Info:[PdfCommand]Completed building documents in 7219.7605 milliseconds.
[19-05-06 06:58:58.661]Info:[PdfCommand.PDF]Start generating PDF files...
[19-05-06 06:59:08.009]Error:[PdfCommand.PDF]Error happen when converting pdf/toc.json to Pdf. Details: System.InvalidOperationException: The file is not a valid PDF document.
   at PdfSharp.Pdf.IO.PdfReader.Open(Stream stream, String password, PdfDocumentOpenMode openmode, PdfPasswordProvider passwordProvider)
   at Microsoft.DocAsCode.HtmlToPdf.HtmlToPdfConverter.SaveCore(Stream stream)
   at Microsoft.DocAsCode.HtmlToPdf.HtmlToPdfConverter.Save(String outputFileName)
   at Microsoft.DocAsCode.HtmlToPdf.ConvertWrapper.<>c__DisplayClass7_0.<ConvertCore>b__1(ManifestItem tocFile)
[19-05-06 06:59:08.197]Info:[PdfCommand.PDF]Completed Scope:PdfCommand.PDF in 9530.6019 milliseconds.
[19-05-06 06:59:08.197]Info:[PdfCommand]Completed Scope:PdfCommand in 17068.4582 milliseconds.
[19-05-06 06:59:08.197]Info:Completed in 17073.8428 milliseconds

Build failed.

[19-05-06 06:59:08.197]Error:[PdfCommand.PDF]Error happen when converting pdf/toc.json to Pdf. Details: System.InvalidOperationException: The file is not a valid PDF document.
   at PdfSharp.Pdf.IO.PdfReader.Open(Stream stream, String password, PdfDocumentOpenMode openmode, PdfPasswordProvider passwordProvider)
   at Microsoft.DocAsCode.HtmlToPdf.HtmlToPdfConverter.SaveCore(Stream stream)
   at Microsoft.DocAsCode.HtmlToPdf.HtmlToPdfConverter.Save(String outputFileName)
   at Microsoft.DocAsCode.HtmlToPdf.ConvertWrapper.<>c__DisplayClass7_0.<ConvertCore>b__1(ManifestItem tocFile)
	24 Warning(s)
	1 Error(s)

stderr: QFont::setPixelSize: Pixel size <= 0 (0)

error: undefined