To determine which pair least matches in respect of computers, let's analyze each option:

1. Megabytes : Bits
   • A megabyte is a unit of digital information storage equal to 10⁶ bytes or 1,048,576 bytes in computer science if measured in binary (2²⁰ bytes).
   • Bits and bytes are directly related units used to describe data size where 1 byte equals 8 bits. Hence, there is a direct relation between megabytes and bits. This pair matches well in the context of computers.
2. PDF : Portable Document Format
   • PDF is an abbreviation for Portable Document Format, which is a file format developed by Adobe to present documents, including text and images, independent of application software, hardware, and operating systems.
   • This pair accurately reflects the meaning of PDF in the context of computers.
3. HTTPS : Hypertext Transfer Protocol Standard
   • HTTPS stands for Hypertext Transfer Protocol Secure. It is an extension of HTTP used for secure communication over a computer network, and its connection is encrypted using Transport Layer Security (TLS) or Secure Sockets Layer (SSL).
   • The option given, "Hypertext Transfer Protocol Standard," is incorrect as it should denote 'Secure' instead of 'Standard'. Therefore, this pair does not accurately match.
4. WWW : World Wide Web
   • WWW stands for World Wide Web, which is a system of interlinked hypertext documents accessed via the internet.
   • The relation between WWW and the World Wide Web is correct and accurately described.

After analyzing all the options, it is evident that the pair HTTPS : Hypertext Transfer Protocol Standard is the least accurate because it incorrectly defines HTTPS. The correct expansion should be "Hypertext Transfer Protocol Secure."

Thus, the correct answer is HTTPS : Hypertext Transfer Protocol Standard.