The phrase "a long document" is correct and usable in written English.
It can be used to describe a document that has a significant length or number of pages, often implying that it may require considerable time to read or review.
Example: "Before the meeting, please ensure you have read the long document I sent you last week."
Alternatives: "an extensive document" or "a lengthy document".
I have a long document and I'd like to create an index for it.
"It can be a long document about what we don't know or a short one about what we know".
So Adam and Shivaun pulled out their laptops and began assembling a long document detailing everything they had experienced.
It's a long document, so do a search (ctrl-f or cmd-f) within the page for PQ4R.
We had a corporate charter, a long document, but it actually made sense, because we did get into some conflicts.
Late last month, Francis issued the first major declaration of his pontificate, an "apostolic exhortation," a long document addressed to Catholics which covers a range of issues.
Lester has a long documented case of the yips, known to baseball fans as the loss of motor function that surrounds a routine movement, like turning from the pitchers' mound and throwing to first base.
"Chris among all things is a member of our community and has a long, documented history of reaching out to children.
He has a long, documented history of saying inappropriate comments at inappropriate times like telling struggling victims of Hurricane Maria in Puerto Rico that they were busting his budget or that Megyn Kelly had blood coming out of whatever.
Donald Trump has a long documented history of flat-out lying, like his claims that Muslims were cheering in New Jersey on 9/11 or that he had proof that President Obama wasn't born in the United States.
Human scabies, due to infestation with the mite Sarcoptes Scabiei var Hominis, affects an estimated 300 million people worldwide per year and has a long documented history of affecting human populations [1].
