property ThumbnailCacheMaxEntries: Integer; // read and write, default 64
The cache stores embedded previews obtained through ExtractThumbnail and rendered previews obtained through RenderPageThumbnail behind a single least-recently-used policy, and this property caps how many entries survive, counting both bitmap entries and cached negative entries that remember the absence of an embedded /Thumb
Retention requires this ceiling and ThumbnailCacheMaxBytes to both be positive; an entry is kept only whilst both ceilings permit it
Reducing the value evicts the least recently used entries immediately, and each dropped entry grows EvictionCount in ThumbnailCacheStatistics
Setting the ceiling to zero disables retention whilst leaving extraction and rendering fully available; every lookup then counts as a miss
Negative values raise EPdfError
The property is guarded by the render lock used by native PDFium page access, so changing it waits briefly for in-flight rendering
Pdf.ThumbnailCacheMaxEntries := 128;
Pdf.ThumbnailCacheMaxBytes := Int64(96) * 1024 * 1024;