Is it proper to use capital letters in (e.g.) document names

Solution 1:

You would not capitalize "standard" because it is not a proper noun.

Think about it this way, if you were to be taking about a city named "San Franciso" rather than a standard named ISO 1923801: The Proper Use of Capital Letters you would not capitalize later instances of the word "city."

I visited the city of San Franciso. I found the city to be delightful.

If you wanted to be very clear in your text, you can give the work an alias, using parentheses.

For example:

I spent a lot of time on the popular standard on capitalization ISO 1923801: The Proper Use of Capital Letters ("the Standard"). Further perusals of the Standard continued to be useful.

You will find this type of device more often in legal documents.

Solution 2:

It is fairly common to refer to a document by a word or a few words "excerpted" from the title in references after the first. This is especially true when several documents are referenced. In this case retain the capitalization of those words that they had in the title. (Often but not always, quotes or italics are also retained.) If you are referring to the document by a description that is not a part of the title, do not capitalize.

For example, suppose you mention "ISO 19238001: The Proper Use of Capital Letters", and also "The Oxford Guide to Capitalization". Later in the text you might refer to these as "In Proper, the committee states that ... On the other hand, Oxford recommends ..."

But if you are using a description like "the standard" or "this book", then no, it should not be capitalized. Capitalization is normally used to indicate a title or a proper name, not a description.