How exactly did you try to find it?
Linux doesn't require ".doc" or any other extensions the Windows does. Putting ".doc" at end of filename just means that is the end of the file name and has no special meaning in and of itself.
That is to say you could save a file as:
So long as its contents, ownership and permissions were the same then it would be viewed as the same type of file by Linux.
Also remember Linux is case sensitive unlike Windows. That means a name ending with ".doc" isn't the same as ".DOC" or ".dOc" or ".doC" etc...
If you know the name of the file and are sure you saved it but aren't sure of the case you can run:
find / -iname <filename>
Where <filename> is the actual file name you think you saved it as.
Alternatively if you aren't sure of the name but are sure of the .doc at end you can try:
find / -iname "*.doc"