Architecture

NetPdf wraps battle-tested, permissively-licensed libraries behind a single coherent surface:

Capability	Powered by	License
Create, merge, split, rotate, encrypt	PDFsharp	MIT
Text & image extraction	PdfPig	Apache-2.0
Render pages to PNG	PDFtoImage (PDFium)	MIT
Barcodes & QR codes	ZXing.Net	Apache-2.0
SVG rasterization	Svg.Skia (SkiaSharp)	MIT

The three entry points

PdfFile (static) — the front door. Create() returns a PdfBuilder for coordinate-based drawing; Open(path/stream/bytes, password) returns a PdfDocument; Merge(...) combines files.
PdfDocument — the unified document model. Reading (ExtractText, GetImages, Metadata), manipulation (ExtractPages, Split, Overlay, Protect, Sign, …), and rendering (RenderPage). It is immutable: every manipulation returns a new PdfDocument, and it implements IDisposable.
NetPdf.Fluent.Document — the auto-layout engine. Document.Create(doc => ...) measures and paginates content with a two-pass Measure/Draw element protocol; custom elements implement IElement.

Page indexes are 0-based everywhere; coordinates are PDF points (1/72 inch) from the top-left corner.
Fonts are resolved from the operating system's font directories on all platforms.
Operation ordering matters: manipulations → Linearize() → XMP / AsPdfA() → Sign() last. See Managing PDFs.