The question asks for the primary advantage of converting an MS PowerPoint presentation to PDF format. Let's review each of the given options:

1. Automatically applies document encryption and password protection: While PDFs can be encrypted and password-protected, converting a PowerPoint to PDF does not automatically apply these features. Additional steps are required to set up such security features after conversion.
2. Ensures universal viewing without requiring PowerPoint software: This is the correct answer. PDF format is widely used and supported across different platforms and devices. You can open a PDF without needing the original software (PowerPoint, in this case). Almost all devices and operating systems, such as Windows, macOS, Android, and iOS, have built-in or easily accessible PDF viewers. This ensures that the presentation can be viewed universally, irrespective of whether the viewer has PowerPoint installed.
3. Enables cross-platform editing with multiple software applications: PDF is generally not meant for editing across different software platforms. Although there are editors for PDFs, the format is primarily designed for viewing and preserving document format fidelity, not for editing.
4. Significantly reduces file size while maintaining visual quality: Converting to PDF can indeed lead to reduced file sizes, but that is not always the case. It's also not the primary advantage compared to universal compatibility for viewing.

Therefore, the primary advantage of converting an MS PowerPoint presentation to PDF format is indeed to Ensure universal viewing without requiring PowerPoint software.