The full version Adobe Acrobat Standard software does not allows me to reformat an existing PDF file to make the page size smaller, however, I can make existing PDF files more accessible to small screens by adding tags to the PDF file (assuming it is untagged). To do this in Adobe Acrobat 7.0, open up the PDF file, then select Advanced > Accessibility > Add Tags to Document.
After adding tags to a document and saving the document, you can use Adobe's reflow option that will cause the document to reflow to fit the size of the screen on which it is opened. This works very well on my windows mobile devices, which run Adobe Reader for Pocket PCs, but it does not work on my Cybook. Adobe Reader for Pocket PCs includes a reflow option in it, the Cybook PDF viewer software does not offer such an option. Unfortunately, many other eInk readers don't have a reflow option within its PDF reader software, so opening tagged PDF files on one of these devices does not reflow the document to fit the smaller eInk screen.
Since you are working with non-DRMed PDF files, another thing you could do is convert the PDF file to a Word document and then change the size of the page from the default 8-1/2 x 11 size to a size that works better on your preferred screen size. Then create a new PDF file (print to PDF). I have formatted many of my own files to fit my Cybook screen size and then printed them to PDF files and it works pretty well.