Learn Hardcore Norwegian: Han laster opp alle nødvendige dokumenter om utdanning og arbeidserfaring som vedlegg i søknaden. - He uploads all necessary documents about education and work experience as attachments in the application.

laste by itself means “to load” (e.g. load a truck).
With the particle opp, laste opp means “to upload” (to a website, portal, etc.).

You conjugate the verb part and keep opp as it is:

Han laster opp … = He is uploading …
Han lastet opp … = He uploaded …
Han har lastet opp … = He has uploaded …

Can opp be separated from laster? For example, could you say Han lastet alle dokumentene opp?

Yes, particle verbs in Norwegian can be split, but with laste opp it’s much more natural to keep them together.

Grammatically possible:

Han lastet alle dokumentene opp. (correct, but sounds a bit marked or old-fashioned)

More idiomatic in modern Norwegian:

Han lastet opp alle dokumentene.

In subordinate clauses, they also usually stay together:

… at han lastet opp alle dokumentene. = “… that he uploaded all the documents.”

Why is it nødvendige and not nødvendig in alle nødvendige dokumenter?

Because dokumenter is plural, the adjective must take the plural ending -e:

Singular: et nødvendig dokument = a necessary document
Plural: nødvendige dokumenter = necessary documents

In Bokmål, for indefinite plural and all definite forms, adjectives typically get -e:

nødvendige dokumenter (indefinite plural)
de nødvendige dokumentene (definite plural)

Why is there no article before alle nødvendige dokumenter? Could I say alle de nødvendige dokumentene?

Without an article, alle nødvendige dokumenter is indefinite plural:

alle nødvendige dokumenter = all (kinds of) necessary documents (in general, whatever is required)

With article + definite forms, you make it refer to a specific, known set:

alle de nødvendige dokumentene = all the necessary documents (those that we both know about / are listed somewhere)

Both are correct; the original sentence talks about “all necessary documents that are required” in a general sense, so the indefinite form fits well.

What does om mean in dokumenter om utdanning og arbeidserfaring, and are there alternatives?

Here om means “about / regarding”:

dokumenter om utdanning = documents about education
dokumenter om arbeidserfaring = documents about work experience

Other possible (but more formal) alternatives:

dokumenter angående utdanning og arbeidserfaring
dokumenter vedrørende utdanning og arbeidserfaring

Om is the most natural and common in everyday language.

Why is there just one om before utdanning og arbeidserfaring?

Norwegian, like English, only needs to state the preposition once when it applies to several items:

dokumenter om utdanning og arbeidserfaring
= documents about education and (about) work experience

You could say om utdanning og om arbeidserfaring for extra emphasis or stylistic reasons, but it’s usually unnecessary and sounds a bit heavy in such a short phrase.

What exactly does som vedlegg mean, and is vedlegg plural or singular here?

Som vedlegg literally means “as attachments” / “as an attachment”, depending on context.

som here = “as”
vedlegg is a neuter noun: et vedlegg = an attachment

The form vedlegg is the same in singular and plural:

Singular: et vedlegg – vedlegget
Plural: flere vedlegg – vedleggene

In this sentence, som vedlegg is understood as “as attachments” in general (plural in meaning), because we are talking about alle nødvendige dokumenter (many documents).

Why is it i søknaden and not til søknaden? Are both possible?

Both can be used, but they emphasize slightly different things:

i søknaden = in the application
- Focus on the documents being contained inside the application (part of its content).
til søknaden = to the application
- Focus on the documents being attached to it or submitted along with it.

In many contexts they overlap. Here, i søknaden works well because attachments are considered part of the application package. Som vedlegg til søknaden would also be correct and maybe a bit more literal about them being attached to the application.

Why is søknaden definite (with -en), but dokumenter is indefinite?

søknaden = “the application” (a specific one: the one he is submitting)
dokumenter (without article) = “documents” in an indefinite sense.

In this context, we know which application we mean (the one he is working on), so it is definite:

en søknad = an application
søknaden = the application

But the documents are not a fixed, previously mentioned list; we are talking about whatever documents are necessary. That’s why they are expressed in the indefinite plural: nødvendige dokumenter.

Is arbeidserfaring singular or plural here, and can you say arbeidserfaringer?

Arbeidserfaring is most often used as an uncountable/mass noun, similar to “work experience” in English:

arbeidserfaring = work experience (in general)

You can say arbeidserfaringer (plural) to talk about distinct, countable experiences, but it’s less common and can sound stylistically heavy:

Jeg har mange ulike arbeidserfaringer.
= I have many different work experiences.

In dokumenter om … arbeidserfaring, it’s the general mass-noun sense: documents about (his) work experience.