You can only read from a cd, not write back any changes made to a file, at least not in same sense as writing back to a hard drive.
You can of course re-write a cdrw, but first have to erase the disc, or write the file to another session, the old data still remaining on the disc in the earlier session to which it was written, so in that sense the file is indeed read only, as it has not been replaced when an updated file is written to the disc.
I think you may need to read that more than once to make sense of it!!
There is a program available called "Clear read only attributes" which will allow you to do a batch conversion of read only files to read/write files useful if you copy a folder full of files from cd to your hard drive. It's avaiable on the net but I'm afraid I don't have the URL at the moment.
I think you can still edit your document from your CD-R without any problem provided that you do not close the session and the end of your work. I have a CD-R in my drive and i use it as you would use a froppy. I can save, delete and edit my document as I wish. Just remember not to close the session after you have saved your document on the CD-R.