Step 1: Recall the full form.
PDF stands for
\[
\boxed{\text{Portable Document Format}}
\]
Step 2: Understand its purpose.
PDF is a file format developed to preserve the formatting of documents regardless of the operating system, software, or device used to open them.
Documents saved as PDF appear the same on all devices.
Step 3: Analyse the options.
• (A) Incorrect. This is not the expansion of PDF.
• (B) Correct. It is the official full form.
• (C) Incorrect. It is not a recognised computer term.
• (D) Incorrect. This is also incorrect.
Step 4: Final conclusion.
\[
\boxed{\text{PDF}=\text{Portable Document Format}}
\]
Hence, the correct option is \(\boxed{(B)}\).