Before you answer, I only want responses for OS X 10.10.3; I have successfully done what I am asking in previous OS's. For some reason, even following the instructions listed in Preview Help, it does not work.

Here is what Preview Help says, and what I have tried to do:
"1. Open the PDFs you want to combine.

2. In each open PDF, choose View > Thumbnails to display the pages of the PDF in the sidebar.

3. Drag the thumbnails you want to add (Command-click to select noncontiguous pages) to the thumbnail sidebar in the other PDF.

4. You can add pages to the end of the document or between pages in a document. Drag thumbnails to reorder them. You can also add a PDF file before or after another PDF document.

5. Choose File > Export to save a new PDF containing all of the combined pages, or choose File > Save to replace the original version of the destination PDF with the version containing the combined pages."

Everything goes well until I try to save the combined PDF. Either by Save, Save As, or Export, only one page of the combined PDF is saved. All the other pages that were dragged into the document do not appear.

In 10.10.3 can you do the above and get a new document with all of the added pages? If you did something else and it works, what?

Thanks.

I haven't done this, but did you try invoking the Print dialog box and saving it as a PDF from there?

Looks like a serious bug to me.

I was able to successfully combine pages following Jon's suggestion.

"...If you did something else and it works, what?"

I have used Combine PDFs without issue in 10.10.3 and find well worth the US $30.00.

Yes, I struggled with that confusing UI this past weekend. What seemed to work was to drag the thumbails over the first page of the first document (the destination) and then a "plus" sign appears. After that I could rearrange the pages.

Very unintuitive, if you ask me.

That's what works for me in 10.6.8. The key is not just dragging a thumbnail to the destination sidebar, but dragging it over an existing thumbnail in the destination sidebar until the plus sign appears.

Ira isn't clear about whether he's tried that specific trick, and the instructions aren't explicit about it.

Ahh, that is the secret! If you drag the thumbnail into the sidebar it still shows a green "+"; but if you drag it onto the thumbnail in the other sidebar, then it is appended properly and can be saved.

Wow, talk about picky.

Made me pretty damned crazy for a while!

I knew that I always saw a plus sign, but it took me a while to realize that it had two different meanings.

Really poor design, not to mention that the help page is inadequate.