Please checkout "How do I export my design to my computer" before moving onto this step.
IF YOU HAVE A MAC
Here are the steps to save multiple images (mood boards) in Preview into a single multi-page PDF.
Select all of the boards that you exported to your personal computer you want in your PDF, right-click and choose open with Preview (this is AFTER you have exported your boards and they are sitting on you MAC hardrive.)
In Preview's sidebar drag the images into the order you want them to appear in your PDF
Then from the "File" menu choose "Print Selected Images" and then "PDF > Save as PDF"
IF YOU HAVE A PC
Please refer to this link if you have Microsoft Office installed on your PC: SAVE AS PDF
You can download the Adobe PDF creator here
For more info regarding combining and merging files to PDF visit the Adobe website here