There are several ways you can do it.
If they are very large, you can zip them up with winzip. When you ftp the zip file to the AIX system, be sure to type binary (or bin) before you do your put. The AIX system has to have Gnu zip on it to unzip it.
You can ftp each one individually from your desktop. You do not need to specify binary when you send them unzipped to the AIX system.
If you have Samba on the AIX system, you can just copy them over, as you would any file to a server you have mapped to your desktop.
Let's say the zipped file or the xls files are in c:\temp.
Go into a DOS box, cd into temp. then type:
ftp <unix system name or IP>
type a user name that can ftp to the AIX system and give the password when prompted.
then type cd /path/to/where/you/want/to/put
then type binary (if you are sending a zip file)
then type put excel.zip
or put excel1.xls
you can also type prompt and then type mput *.xls to send all files with .xls in the c:\temp directory to the directory on the AIX system.