If you have already converted all PDFs to JPEG and then stored them as JPEGs in the DB, then TfrxOleView will be able to display them.
If not, you need to create a custom report component derived from TfrxReportComponent and handle its Draw event. The Draw event should get the PDF blob, convert it as a metafile/bitmap, and then draw the metafile/bitmap on the canvas of the custom control. This custom component needs to be used in your report instead of the TfrxPictureView control. (Check FastReport's Developer manual for more information.)
You will need a PDF library to convert the PDFs to metafile/bitmap. (I recommend my company's product PDFtoolkit for this.)
PDF-to-JPEG conversion takes time and may slow down your reports. So, I would advise you to first run the conversion on your DB and store the images in another column. That way, you don't have to write any custom report component. You just refer the images column/field.