Hi,
DSpace 6 doesn't tend to use dashes or underscores in any URLs except it will display the name of your files (e.g. "my_file.doc" would appear in a file download URL like /xmlui/bitstream/handle/1234/56/my_file.doc ).
As of DSpace 7, however, all URLs use dashes, as they are based on unique identifiers (UUIDs). So, download links look more like /bitstream/11111-22222-3456789/download
In all honesty though, I don't think either approach has a big change in SEO. The DSpace team has regular contact & feedback with the team at Google Scholar, and they have never reported file names as being a major concern about SEO of DSpace. Instead, Google
Scholar has helped us write up SEO instructions specific to DSpace 6 here:
A similar guide also exists for DSpace 7:
Tim