function ExtractThumbnail(PageNumber: Integer): TBitmap;
| PageNumber | Integer. One-based page number. A number outside the document returns nil. |
| Result | TBitmap. The embedded thumbnail as a bitmap the caller owns and must free, or nil when the page stores none. |
PDF pages may carry a /Thumb image so a viewer can show a page preview without rasterising the page. This decodes that image and hands it back as a bitmap
It works on any page, not only the current one, and does not change PageNumber: the page is taken from the internal page cache when it is not already loaded
A nil result means the page has no embedded thumbnail. To get a picture in that case, render one with RenderPageThumbnail