Here are steps on how to save multiple images as one PFD file on a Mac.
– Open the first image in Preview
– If not already turned on, turn on Thumbnail view (View -> Thumbnails)
– Select the rest of the images and drag them onto Thumbnail view
– Select Print from File menu
– Click on PDF drop down in lower left corner, and select Save as PDF
– Select save location and save the file (choose “use .PDF” if prompted)