I am sorry to say that dompdf does not yet fully support Bootstrap. If you want to stick with dompdf it might be easier to use tables since the content is conducive to that type of structure.
That being said, I think you can get this to work. There appears to be a bug in handling the :before/:after content. When the content is empty or a space dompdf seems to discard it (or something to that effect, haven't investigated yet). The following additional styling seems to get things looking a bit more as expected.
.row-fluid:after {
visibility: hidden;
content: "."
height: 1px;
clear: both;
}
.span6 .pull-right {
display: none;
}
I hid the action buttons (the .span6 .pull-right styling) since those don't really seem necessary in the print-out and I'm not sure how well floats inside floats work at present. There is one last issue that is present, and that's that dompdf seems to lose track of the layout after a certain number of floated elements. I'm not sure right now how to best address that.