Is there any way to batch convert .jpg files into single .pdf files?
Never forget your little fellow Automator, sitting in a hard drive near you :)
Launch Automator, create a new workflow. Now you need to add two actions:
In the "Files & Folders" select Ask for Finder Items and drag it.
In the PDFs Category, find "New PDF from Images" and also drag it.
Your workflow should look like this:
Now read the options for each Action and customize accordingly. Investigate similar actions (especially in the Files & Folders category) and perhaps the Photos if you want to transform your images from JPG to something else. In fact, go ahead and look all over automator, there are a lot of interesting things there. :)
When you're satisfied with your Workflow, click "Run" on the top right corner. Watch the magic happens and depending upon the number of items and your CPU power, you might go grab a coffee/tea.
I'm more at ease with commandline tools myself. ImageMagick is great for this. You can install IM through Homebrew:
brew install imagemagick
After that you can simply do this in a Terminal:
convert *.jpg output.pdf
and if the resulting PDF is a bit too big you can try:
convert -quality 60 *.jpg output.pdf
Of course ImageMagick also works on other Unix systems, and even on cygwin.