Which version of Acrobat are you running, and on what operating system? Can you convert the DWF file to PDF by just selecting File>Create>PDF From File? Are you getting any errors when doing this?
Windows 7 using Acrobat Pro XI
When i try and convert the dwf to pdf like u say, i get a pdf but only half the drawing showing.
That's odd, but that may related to the problem you are having when combining files: Either the algorithm used to read from DWF files has a problem, or there is something wrong with the drawing itself. I don't recall what settings you can change when converting DWF (I am on a Mac right now, and the Mac version of Acrobat does not support this), so just open up Acrobat's preferences, go to the "Convert To PDF" category and select "Autodesk AutoCAD" to see these settings. You may want to try to modify these settings to see if you get better results.
You should be able to convert to PDF from within AutoCAD, is that an option?
If this does not fix your problem, unfortunately, the only option is to report a bug: Feature Request/Bug Report Form