Hold down Option key when clicking on "Save Combined PDF..." button to bypass the save dialog.
PDFCombo version 1.0.2 adds two new features:
Now you can print to PDFCombo from any app by including it as a target in the Print > PDF dialog. See the section "To add an item to the PDF pop-up menu" in this Apple Guide on how to print PDFs directly to applications like Evernote and PDFCombo. Use this technique to collect several printouts into one PDF such as when collating a web article split over multiple pages into one PDF, or to pack several related web articles into a PDF.
Hold down Option and click on "Save Combined PDF" button to bypass the save dialog and create a PDF in the temporary directory.
PDFCombo is a free app for Mac OS X 10.6 and later, available from the Downloads page.