After you select ALL the PDFs, you will see something like the image below. Note how when you select multiple files the first one shown is the last one that was opened, therefore you can only make one PDF with PDFs in Preview — I think... — Apple Support What works in Preview, and how does the Adobe Image Processor (AIP) work in Preview? How can I use multiple PDF document in Preview in OSX El Capitan? June 24, 2023 — What does it mean with preview in OS X El Capitan? If you click on a file in Preview that you want to use in another program, you will get the following dialog box. The “Open” function in Preview has been changed so that if you select a document and press the “Open” button, the open document is placed in an application's main window. The Document Picker (left), which displayed open documents in the lower left portion of the interface, has been renamed to “File Picker”. To create documents in Preview, select a document with the document picker, and then press the “Open” button (see the following example). When you see a pop-up window that says “Your document was opened”, that indicates a Document Picker that you made and can now place your document anywhere you want, or even create multiple versions of your documents. The document picker can display a small number in the upper left corner called “Preview Number”. This number indicates the number of documents that are displayed in this view. In our case, the file numbers 4 and 5 have been filled out. Why is the document picker not showing up? The File Picker (left), where you select files to open with Preview in OS X El Capitan, has been renamed to The “File Picker”. When you select a document with the document picker, its name is displayed in the lower left portion of the window, and you can select it to place it in one of the applications, “File” or “Other”. When you select a file and press the “Open” button, the open document (in the example above) is placed into your primary application's window. In the example above, we have selected File > Create and File > Other in the File Picker.