However, you can select the files by clicking the checkboxes in front of the files you want, then clicking the "More" button and selecting download. You'll get a dialog box asking you what TYPE of file you want. For our purposes, you'd select "PDF". Once you click "OK", Google will combine the files into one ZIP file and download it to your MAC's "Downloads" folder. Here's where the Automator comes in.
You'll, first, want to expand the ZIP file by double clicking on it. The extract program will create a file folder with all the individual PDF files in it.
Select the PDFs you want to combine into a single file by Command Clicking on noncontiguous ones, Shift Clicking on a group of files; or, if you want them all, just Select All (Command + "a").
Now, click and drag those files into the upper Automator window ("Get Specified Finder Items").
Open the file in Preview and click the top left icon in the Preview window to get to the Thumbnail view.
You'll want to quickly check through the file to look for blank pages and the like. You can select a page in the thumbnail sidebar and hit your DELETE key to get rid of it. When you close the file it will automatically be updated. Here's a link to the Automator workfile.