This guide will walk you through to process of creating a multipage PDF document from multiple image files (png,jpg,etc).
1. Find a Screen Shot or Image File
After taking your screenshots or organizing your image files find one file and open in Preview (File –> Open With –> Preview). For most people Preview is the default application for opening images, which means you can also double click on the file to open it.

2. View in Preview App
Make sure youre image opened properly in preview.

3. Create a PDF File
In order to add multiple images together into a PDF we need to create a PDF file. Right now you only have an image file, which does not display pages or allow multiple files to be combined together. Navigate to the menu bar and select File –> Export as PDF

4. Name the PDF and Choose a Location to Save to
Give an appropriate name for your PDF. This should be what you want the final project or filename to be.

5. Open PDF and Rotate
Often when a PDF is created from an image it will rotate it incorrently. In the center of the Preview window there is an option to rotate the current image, do this until the image is rotated properly.

6. Change View to Contact Sheet
Locate the view picker in the upper left corner of the window and then select contact sheet.

7. Drag a Second Image Into the Document
Find the next image that you want to add to the PDF and drag it on top of the image in Preview. If you do not drag it directly on top of the existing image Preview just acts as though you are trying to view 2 completely separate documents and it does not add the image to the existing document.

8. Make Sure There is Only One Document
Just to double check that the last step was completed properly make sure that the document title says page 2 of 2. If it says that there are multiple documents you must delete that image and repeat step 7.

9. Repeat the Process(if necessary)
Repeat step 7 as many times as you need to complete your document. After you have added all of your images, navigate to the menu bar again, File –> Save. You should now have a multipage document made from images or screenshots.
