PDF Generation

**Describe the bug**
PDF generate isn't located where I believe it'd be after following the documentation.

I might have misunderstood something, so I hope somebody could help.

**To Reproduce**
```
Here's my docfx.json
{
"metadata": [
{
"src": [
{
"files": ["**/*.csproj"],
"exclude": ["**/bin/**", "**/obj/**"],
"src": "../"
}
],
"dest": "api"
}
],
"pdf": {
"content": [
{
"files": [
"articles/**.yml"
],
"exclude": [
"api/**",
"**/bin/**",
"**/obj/**",
"_site_pdf/**",
"**/toc.yml",
"**/toc.md"
]
}
],
"resource": [
{
"files": [ "articles/images/**"]
}
],
"overwrite": "specs/*.md",
"dest": "_site-pdf"
},
"build": {
"content": [
{
"files": [
"index.md",
"articles/**/*.md",
"articles/**/*.yml"
],
"exclude": [
"_site/**",
"**.*.pdf"
]
},
{
"files": [
"api/**/*.md",
"api/**/*.yml"
]
}
],
"resource": [
{
"files": [
"articles/images/**"
]
}
],
"output": "_site",
"template": [
"default",
"modern"
],
"globalMetadata": {
"_appName": "BKS.Gmud",
"_appTitle": "Gmud",
"_enableSearch": true,
"homepage": "index.md",
"pdf": true,
"pdfTocPage": true,
"pdfFileName": "Manual-GMUD.pdf"
},
"postProcessors": ["ExtractSearchIndex"]
}
}
```

**Expected behavior**
After running ```docfx pdf````, I though a folder ```_site-pdf```` would have been generated.

**Screenshots**


It was generated under _site/articles/

**Context (please complete the following information):**
- Docfx version: 2.77.0+bd00e2b93951e9e7fa6e5abd990d2cd77d7a83bd