CD-R's are in normally once written and then can only be read.
CD-RW's are different.
If you have copied the document to a normal CD-R then the file is read only because it cannot be changed. A floppy disk has a write-protect tab so you can make files read-only on them by simply moving the tab to Protect. A CD does not have this and, as mentioned above, once the data is written on a CD, it cannot be changed and re-saved on the CD.