LoadCustomDocument method

LoadCustomDocument is the streaming counterpart to the buffered LoadDocument overloads. It is backed by PDFium's FPDF_LoadCustomDocument entry point through the TPdfStreamAdapter helper class introduced in v1.8.0 PDFium pulls blocks from AStream only when needed, so the stream is not copied into memory: a TFileStream over a 4 GB PDF, an HTTP-backed TStream, or a database BLOB stream all become usable without staging the entire payload in RAM first.

AStream must support seeking. Its lifetime must extend until the document is closed (call UnloadDocument or set Active back to False). Pass AOwnsStream = True to hand stream ownership over to this TPdf — the adapter then frees AStream when UnloadDocument runs. Pass False when the caller wants to keep the stream alive after the document closes.

LoadCustomDocument is intentionally a separate method (not another LoadDocument overload) so callers passing a TMemoryStream subclass cannot accidentally bind to the buffered LoadDocument(TMemoryStream, Boolean) overload — the Pascal argument shape is identical but the semantics are very different (buffered vs. on-demand).

• Set Password before calling LoadCustomDocument when opening encrypted documents.
• The TPdf does not assume anything about AStream — it can be a TFileStream, a TMemoryStream, or any TStream subclass that honors Seek and Read.
• PDFium reads small chunks repeatedly, so very slow streams (network shares, remote files) will translate into noticeably slower page operations than a buffered LoadDocument over the same data — choose based on memory pressure vs. latency.
• Closing or freeing the stream BEFORE the TPdf releases it produces undefined behavior at PDFium level. When in doubt, transfer ownership with AOwnsStream = True.