If they're "zipped" files (usually .zip extension), were you careful to upload them in "BINARY" mode? Do not force "ASCII" mode -- that will definitely corrupt the files.

Many FTP clients give you a choice of forcing ASCII or BINARY mode on transfers, or letting the system decide for you. Some only let the system look at the file and make the decision. BINARY mode sends everything unchanged, while ASCII mode translates the byte(s) that mark the "end of line" between what they are on the PC and the server. Sometimes there are other options, such as converting to and from EBCDIC (IBM mainframes). If you're not doing anything to specify or force one way or the other, but letting the system decide for itself, there shouldn't be any problem. If you're forcing ASCII, your files will be corrupted. Have you tried downloading them yourself, using the browser (or an FTP client) the way a user would, without explicitly specifying ASCII or BINARY, and seen if the files are corrupted?

1. What are you using to upload your files? An FTP client (which one)? cPanel > File Manager?
2. Are the files with ".zip" extension on your PC?
3. Is your PC Windows, Mac, Linux, or something else?
4. Is your hosting plan Basic or Business (Linux based), or Windows?
5. How are they accessed by a visitor... a link (<a href="http://...>) or something else?
6. Do you have anything in your .htaccess file(s) defining what a .zip file is? (AddHandler, AddType) Perhaps you have the wrong thing set, and it's telling the system to treat the file as ASCII.
7. If you download the file via FTP, in the same manner as you uploaded it, is it corrupted?

1. I use the file manager that is located in my cpanel.
    I don't believe that gives you a choice about whether to use ASCII or BINARY mode -- it decides automatically.

2. The files that I upload onto my cpanel are in zipped format.
    My specific question was, do the zipped files have an extension of ".zip"? If you have something else, like ".txt", the file uploader might  be looking up common file extensions to decide whether to use ASCII or BINARY (and if all else fails, poke around the file to guess which one to use). You didn't rename a zipped file before or after uploading, did you?

3. I use a PC 
    Windows, I presume. In text (human-readable) files, lines end with carriage-return-line-feed (x0D0A).

4. My host plan is Basic
    Which is Linux, where text file lines end with a newline (x0A). This is where the ASCII conversion (if done) takes place.

5. I copy and paste the http:// links onto the emails.
    So your link is something like http://www.yoursite.com/zipfiles/a_zip_file.zip? Did you include the ".zip" file extension in the file as you uploaded it, and in the link?

6. I checked the file manager and there are five .htaccess files.
    The first one (.htaccess) is the current, active one. It appears to have been created by using the cPanel button to enable PHP 5. It should not affect zip file processing. The others (.htaccess-date-time) are backups, and are inactive. Note that in one of them you had activated a redirect, and it's no longer being done (you'll have to add it to the current .htaccess again).

7. I have downloaded the file that was supposedly corrupted using the file manager and it worked fine for me.
    Well, that's interesting. So you can download the file back to your PC and it's OK, but if you access it via a link, it's corrupted? Can you show us just what the link (code) looks like?

One other thing... in cPanel > File Manager, it gives you the option to unzip a zipped file that you've uploaded. You didn't do that, did you? You want to leave the file in its zipped state.

I just clicked on your link, downloaded and saved the zipped file, unzipped it, and looked at the two .pdf and one .doc file within it. Everything seems to be fine. I'm running Windows XP Home SP2. The file seems to be on your site correctly, so it must be user error in downloading it (that, or Vista is screwed up royally).