with preview solution:
The first step is to create an application that concatenates PDFs. This is much easier than
Start up the Automator application
Automator will ask you to “choose a template for your workflow.” Pick the one named
Drag and drop Actions from the menus on the left to create a workflow that looks like this:
From the Automator, save this workflow somewhere. You now have an application that
Optionally, put this application in your Pictures folder, or put it somewhere else and make a symlink to it in your pictures folder.
Now that you have a PDF-concatenator application, you can use it from the Finder by selecting the PDFs that you want to concatenate and then dragging-and-dropping them onto the app.
When you scan documents from the Printer/Scanner app, they end up by default in your Pictures folder with names like Scan.pdf, Scan 1.pdf, etc. I created an empty file called Scan.pdf in that directory to make sure that all my actual scans would be named sequentially as Scan 1.pdf, Scan 2.pdf, and so on.
Based on this setup, my procedure for scanning a multi-page document into a single PDF using the flatbed scanner is:
Open up the Printer/Scanner window and use it to scan each page.
In the Finder, go to the Pictures folder.
Select the PDFs representing the scanned pages.
Drag and drop the selected files onto the PDF-concatenator application’s icon.
The combined document will open in Preview. From Preview, save it as whatever file name you like.
That last step–saving the file from Preview–is essential. The Automator action that does the actual concatenation puts the temporary file in a randomly-named file under /var/folders. My automator script, though, moves this temporary file to the Trash after opening it in Preview. If the document you’ve just scanned contains sensitive data, such as a bank statement, I recommend that you 1) from Preview, save your document someplace secure, like an encrypted disk image, and 2) from the finder menu, use the Secure Empty Trash operation to delete the temporary file.
other way: adobe Acrobat for mac but its way to expensive...