他 for a person, or more specifically, a male person
她 for a female person
牠 for an animal
它 for an inanimate object
祂 for a deity
A document is inanimate, so 它 is the appropriate pronoun to use. Having said that, it is not always necessary to translate the "it" in an English sentence. If you'd give us the context, we may be able to provide a better ...
In general "让" means "Let"
"奥运会可以让世界认识中。" = Let...known
妈妈让弟弟做作业。 = Let
上课的时候，老师不让 我们 说。= Don't let.
The "Let" word in English, we may use it to ask for permission. However in Chinese, not only it can be used to ask for permission, it can also be used to describe a consequence of an act (Made me to).
Eg. 他让我用他的车 = He ...
In Chinese, an adjective can be used as predicate, and does not have to be accompanied by the Be verbs (am/is/are) or similar verbs (become/turn/...).
她(She)很(very)漂亮(beautiful), "is" is omitted after "she".
花儿(Flowers)红了(red), "turned" is omitted after "flowers".
我们(We)不(not)一样(the same), "are" ...
When 只 is used after 一，it is always interpreted as a measure word.
If you want to say "A video for you only", it will be "一段只给你的视频", or "只给你的视频".
Also the character 只 as a measure word evolves from the action of catching a bird:
Therefore it is mostly used on animals:
只 zhi1 (traditional 隻), a classifier, is completely separate from 只 zhi3 (traditional 只), meaning "only".
一只 is the former and cannot have the meaning "for you only".
只 is not a classifier I've ever seen used for videos, so I'd guess it was either used in error or the author's use of artistic license to make the video seem "cute"...
Both 只 and 支 could be measure words and sometimes can be interchangeable.
兩支光纖模組 兩只光纖模組(Optic module for network device)
From my experience, 只 is commonly used in formal documents, such as contract, quotation, etc.
For objects which is long and straight, you can only use 支 instead of 只.
The '的' in '自己的' is a possessive marker. "自己" = "oneself"; "自己的" = "onself's"
Some people use 的 and 地 interchangeably in the adverb role. The second 的 in the second sentence functions the same as 地 as an adverb marker
The term '努力' itself can function as an adverb. "...