This is caused by the fact that images are exported without compression. We plan to export the images with DCTDecode, which will decrease the size of the images drastically. The feature is currently scheduled for 2015 Q3, expected in October. You can vote for it and track the development progress at the public portal here:
RadRichTextBox: Export images to PDF with DCTDecode.
Possible workaround with the current version would be to change some of the export properties
PdfFormatProvider provider =
PdfExportSettings exportSettings =
exportSettings.ContentsCompressionMode = PdfContentsCompressionMode.Automatic;
exportSettings.ContentsDeflaterCompressionLevel = 9;
exportSettings.FloatingUIContainersExportMode = PdfInlineUIContainersExportMode.Image;
exportSettings.ImagesCompressionMode = PdfImagesCompressionMode.Automatic;
exportSettings.ImagesDeflaterCompressionLevel = 9;
provider.ExportSettings = exportSettings;
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