I have used PDF24 for quite a long time to convert scans of my musical scores into PDFs used by forScore, a popular sheet music organizer that I use on my iPad. This process used to be very quick and reliable; just click-drag multiple JPGs over to the sorting window, compress into one row, then save. Boom!
Unfortunately, the process is no longer reliable. 🙁 Now at least half the JPGs do not display correctly in the sorting window - in fact, they compress into a thumbnail occupying only one-ninth of the page (one-third X axis, one-third Y-axis). Sometimes, this thumbnail is repeated in three, but again only taking up one-third of the page height. The way I get around it is to open the individual, offending JPG from within the image viewer IrfanView, print out to PDF, and then to add the single PDF among the various JPGs for overall compiling.
I apologize if my terminology is incorrect. Please let me know if you need rephrasing, or perhaps a screenshot set would illustrate better. TIA...