Add XML documentation for Android.Graphics.Pdf

The `Android.Graphics.Pdf` namespace contains **176** `To be added` placeholders in source XML documentation (excluding `docs\xml\index.xml`).

Example command:

```powershell
rg -n -i --glob 'Android.Graphics.Pdf/**' --glob 'ns-Android.Graphics.Pdf.xml' 'to be added' docs\xml
```

## Update log

- 2026-09-10: Refreshed the placeholder scan from **156** to **176** (increased by **20**). Related documentation PRs: [#644](https://github.com/dotnet/android-api-docs/pull/644).

Start by running the provided rg command against docs\xml to enumerate the 176 placeholders under Android.Graphics.Pdf, then inspect the matching source XML documentation entries and the namespace index where needed. Done means each listed “To be added” placeholder has accurate XML documentation and a repeat scan finds none in the namespace.

Written by the indexing model from the issue text.