According to what I have just looked up, .opf is only a metadata file and does not contain the actual book.
Certainly, all the .opf files I have were created by Calibre when looking at (nonDRM) .epub books.
How big are your .opf files?
Do you have .epub files areound for those books as well? Try opening those with ADE.
Some filetypes (eg .acsm) won't open in ADE using the file open (ctrl-O) option, but will open using drag drop from Explorer onto ADE.
You could try drag-dropping your .opf files ~~~ drag-dropping mine doesn't work.
Use ADE all the time to open .epub files.
In trying to open .opf files I tried both ways - rht click on file in folder and select open with and use ADE as well as within ADE trying to use file open from the menu. Neither worked.
Have you checked to see if the .opf files are the 'real' book or just metadata files, and to see if you have associated normal (maybe DRM) .epub files?
The .opf file is in the same directry as the HTML chapters. The name of the file is "content.opf" for both books that I have downloaded form Kobo books.
What is the directory structure immediately above the directory containing the HTML chapters and the 'content.opf' file?
You usually see the structure you describe inside a .epub file (an .epub file is just a zipped up directory).
It may be that you have acidentally navigated inside the directory structure of a .epub file without realizing. In that case, try opening the .epub file in ADE.
If not, try packaging the entire directory as a zip file, rename it to .epub, and then try to open it with ADE.
Received the file as a zip file from Kobo. Tried the renaming bit .zip to .epub and it worked!
Ah, excellent - thanks for telling us. It is very likely that the same issue will come up again in the forum. Seems an odd way for Kobo to distribute the file, but there you go.
Looking up 'kobo .zip' on google shows you are not the first to have the issue. If you get any more Kobo oddities, you may find some help at http://goodereader.com/forum/showthread.php/3946-The-Ultimate-Kobo-Tips-amp-Tricks-Thread