How to password-lock a file?

You can put your file into a password protected disk image file. You create a disk image file with the Disk Utility app in the Applications/Utilities folder. These instructions are from the Disk Utility Help File:

To protect documents in a secure disk image:

1. Choose File > New > Blank Disk Image.
2. Type a name for the image, and then choose where you want to save it.
3. In the Name field, type a name for the disk that appears when you open the disk image. The disk and the disk image can have the same name.
4. Choose the size of the disk image from the Size pop-up menu. Be sure you choose a size that’s large enough to hold any changes and new documents you might add.
5. Choose an encryption option from the Encryption pop-up menu.
6. Use the default settings for the rest of the options: Choose Mac OS Extended (Journaled) from the Format pop-up menu. Choose “Single partition - Apple Partition Map” from the Partitions pop-up menu. Choose “read/write disk image” from the Image Format pop-up menu.
7. Click Save.
8. Enter a password, deselect “Remember password in my keychain,” and then click OK. Disk Utility creates the disk image and mounts its disk on your desktop. IMPORTANT:If you select “Remember password in my keychain,” anyone with access to your computer may be able to open the disk image without entering the password.
9. Copy to the disk the documents you want to protect
10. If you want to erase the original documents so they can’t be recovered, drag them to the Trash and choose Finder > Secure Empty Trash.

To access the documents in the disk image, double-click the disk image and enter the password. A disk with the documents appears on your desktop.

You could also use a ZIP utility to create a password protected zip file. Here is one example: iZip for Mac

I doubt this will this work for sending an e-mail attachment or for use on a download site. This is what the OP wants.

Why would this not work?

To the best of my knowledge there is no way to password protect a "file" for email or upload using just the OS.

While the minimum size of a disk image is 10 megabytes this is not usually not too large for most email servers. I had no problem with verizon. If it is a problem you can do the following:

1. Create a password protected disk image per the original instructions.
2. After you have copied the files to the dmg and unmounted it you can compress it using Finder > File > Compress "filename" to reduce its size.

I also suggested using a ZIP utility to create a password protected zip file.

Another alternative is to use a "download site" that has an encription/password option on there end. But that is usually and added feature in "premium" plans.

Yes, that works, albeit very slowly. Also, zipping it hardly compresses it any further. Zipping it goes from 10.9 to 10.8MB. Minimum is 10.5MB. Suppose it will do in a pinch. But I wonder if it would be Windows friendly?

There's this for creating password a protected zip. But it's not simple.

http://www.betalogue.com/2010/03/31/encrypted-zip/

Use GPG.

Please explain: what was the reason for using Ez7z and not the native OS X zip/compression?