I think you have little choice but to use an ftp app unless your client had direct access to the hosting company’s file manager. By having direct access, your client would simply upload the revised PDF from time to time, overwriting the previous one. As long as the pdf file has the same name, everything should work fine.
As an alternative, you could create an ftp account for your client, but one that limits access to just the directory where the PDF is stored. You could then set up a free ftp app on the client’s computer and configure it for him to access the account you’ve created. This could be saved as a default bookmark. All the client will then have to do is fire up the ftp app, select the bookmark connection and drag the PDF file into the upload window. It may take a few minutes of training, but the process is simplicity itself. If you set the account up correctly, your client will be unable to stray outside of the designated directory, thereby giving very good damage limitation if something doesn’t work.
Another option that I’ve used for clients who simply can’t handle FTP or file manager, is to create a hidden html form in a password protected subdirectory on the site. The form itself contains a file upload field and gets passed to a script on the server. The script I commonly use is the free Tectite Formmail script. This script can be configured to do a number of things, such as limit the size of uploaded files, rename uploaded files, and send the uploaded file to a specific file repository location on the server. All these features make a simple and foolproof file replacement option that clients can use easily. The simply enter the URL of the upload form, supply the designated password and add their file to the upload field. The script does everything else. You can find instructions on how to configure the script HERE.