If you want to make a single PDF from many JPGs.
You can do the following:
1.)Open "Automator" and select "Workflow".
2.)Choose "Files & Folders" from the left hand column and then drag "Get Specified Finder Items" to the right hand column.
2.)Type in "Preview" in the search field.(See the diagram below.)
3.)Drag "New PDF from Images" to the right hand column. Like the following:
4.)Click "Add" to select the image files that you want to add the pdf file.
5.)Enter the name of the output file in "Output File Name".
6.)Click the "Run" at the top right hand corner.
That's it ! Easy right !