I cannot open PDFs (and I guess other files) with spaces in filenames in Windows 7 with JabRef 2.6, but the same works fine in MacOS X 10.6.
For example, when attempting to open the file "By topic\Computer Vision\Stereo\BleyeRK2010.pdf", I get the foallowing error message: "Windows cannot find 'E:\[...]\Reading\By" "topic\Computer" "Vision\Stereo\BleyeRK2010.pdf'. Make sure you typed the name correctly, and then try again.
Note that the spaces are essentially replaced by "doublequote-space-doublequote", and the entire path is put in single quotes.
I experience the same with JabRef ver. 2.7.2 on Windows 7.
When a pdf file contains space(s) in the filename, after clicking on the file icon in the list of entries, the following happens:
* if in "external program settings", the "default application" option is set for pdf files, status "External viewer called" appears in the status bar and nothing happens;
* if a pdf viewer application is explicitely set by its filename in the "external program settings" for pdf files, the pdf viewer launches and returns an error related to incorrect filename or non-existing file.
Same with JabRef 2.7.2 running on Windows XP SP3: If the PDF filename contains spaces, multiple instances of the PDF viewer are called (one for each "part" of the filename).
Using the same files (including the .bib file) and the same JabRef version on Linux works.