AES Crypt is free file encryption software you can download at https://www.aescrypt.com/download/. AES Crypt allows you to right click on any file, and enter a password that will encrypt the file.
If a recipient of the file also has AES Crypt installed, they'll be able to open with the encrypted file with the right password. The encrypted file will be extracted to the same folder in which the .aes file to located so long as a file the same name does not already exist in that folder.
On the same web page you can download the console version of AES Crypt, which is a single file, aescrypt.exe in a zip file. If you put this file in a folder with multiple files and run these commands in a batch file:
aescrypt -e -p abc^^123 C:\FooFolder2\Book1.pdf aescrypt -e -p abc^^123 C:\FooFolder2\Book2.pdf aescrypt -e -p abc^^123 C:\FooFolder2\Fielding.pdf aescrypt -e -p abc^^389 C:\FooFolder2\Pitching.pdf
. . . you will be able to encrypt individual files that you specify en masse. Switching the '-e' to '-d' will let you decrypt multiple files.