In PDF 4 and 5 Pro, in order to edit a PDF made from jpg or any other image file, you first need to go to the toolbar and click on "Convert PDF". You'll get a drop down list that includes "Searchable PDF" (second to last choice). Select this and PDF Pro's built in OCR goes to work. When it's done (and it can take a long time if you're converting a really big PDF) you'll be prompted to save the output under a new file name. I'd do this as there are some PDFs that this doesn't work on - mostly, I've found, if the pictures have been rotated during PDF creation. You can get everything back to normal... eventually! If you experiment. This is one of the reasons I'm interested in version 6 because you can apparently do this to pages or even to blocks of text. In PDF 5 Pro and earlier, you have to perform the conversion on the entire document - selecting a single page out of 100 would allow me to find the right orientation for the image much more quickly.
Hope this helps,