What is correct abbreviation for electronic copy in a letter?

There isn't a universally accepted abbreviation for "electronic copy" in a letter. However, here are some options depending on the context:

Formal:

* Elec. copy: This is a concise and widely understood abbreviation.

* E-copy: This is more informal but still acceptable in many contexts.

* Electronic version: This is the most formal and unambiguous option.

Informal:

* E-copy: As mentioned above, this is a good option for informal communication.

* Digital copy: This is a more descriptive option that clearly specifies the type of copy.

Specific Cases:

* PDF: If you're referring specifically to a Portable Document Format file, use "PDF."

* DOC: If you're referring to a Microsoft Word document, use "DOC."

Ultimately, the best option depends on the specific letter and your audience. Consider the level of formality and clarity required in your communication.