Dodajte vlastoručni potpis na zadnju stranicu.

Yes. Dodajte is the imperative form of dodati, meaning add.

In this sentence, it is used as an instruction:

• Dodajte ... = Add ...

It can address:

• you (plural), or
• you (formal singular)

So this sounds like a polite or official instruction, which is very common in forms, procedures, and office language.

The base verb is dodati, which means to add.

A few useful forms:

• dodati = to add
• dodam = I add / I will add
• dodajte = add! / please add

This verb is perfective, so it usually refers to adding something as a completed action.

Potpis means signature.

Vlastoručni potpis is more specific and formal. It means something like:

• handwritten signature
• signature written by your own hand

This is often used in legal, administrative, or official contexts to make it clear that a real, personal signature is required, not just a typed name or some other mark.

So:

• potpis = signature
• vlastoručni potpis = handwritten/personal signature

Vlastoručni literally means written by one’s own hand.

It is made from:

• vlastit / vlast ideas related to one’s own
• ruka = hand

In modern usage, learners should mainly remember it as a fixed formal adjective meaning:

• handwritten
• personally signed
• in one’s own hand

You will often see it with nouns like:

• vlastoručni potpis = handwritten signature
• vlastoručno napisano = handwritten

Because after na in this sentence, Croatian uses the accusative case.

The phrase means movement or placement onto something:

• na zadnju stranicu = onto the last page

So both words change into the accusative singular:

• zadnja → zadnju
• stranica → stranicu

Compare:

• Zadnja stranica je prazna. = The last page is empty.
  Here it is the subject, so it stays in the nominative.
• Dodajte potpis na zadnju stranicu. = Add the signature to the last page.
  Here it follows na with motion/direction, so it becomes accusative.

The preposition na can take different cases depending on meaning.

• accusative = movement toward / onto a place
• locative = location at/on a place

Here the idea is putting or adding something onto the page, so Croatian uses the accusative:

• na zadnju stranicu = onto the last page

Compare:

• Potpis je na zadnjoj stranici. = The signature is on the last page.
  This is location, so locative is used.
• Dodajte potpis na zadnju stranicu. = Add the signature to the last page.
  This is direction/placement, so accusative is used.

Often, yes.

Both can mean last:

• zadnju stranicu
• posljednju stranicu

In many everyday contexts, these are interchangeable.

However, there can sometimes be a slight nuance:

• zadnji can also mean the one at the back or final in order
• posljednji more clearly means final / last

In this sentence, zadnju stranicu is very natural and common, and most speakers will understand it as the last page.

Croatian word order is fairly flexible, but the original version sounds natural and standard:

• Dodajte vlastoručni potpis na zadnju stranicu.

Other versions are possible, for example:

• Na zadnju stranicu dodajte vlastoručni potpis.

This version puts more emphasis on where to add it.

You could also hear:

• Dodajte na zadnju stranicu vlastoručni potpis.

That is understandable, but the original sentence is the smoothest and most neutral for an instruction.

Yes, definitely.

Several things make it sound formal or administrative:

• dodajte as a polite imperative
• vlastoručni potpis as formal legal/official vocabulary
• the whole structure sounds like an instruction on a form or in an office

In casual speech, someone might say something simpler, such as:

• Potpišite zadnju stranicu. = Sign the last page.
• Stavite potpis na zadnju stranicu. = Put your signature on the last page.

But the original sentence is very appropriate in official writing.

A simple pronunciation guide is:

DOH-dai-teh vlah-stoh-ROOCH-nee POHT-pees nah ZAD-nyoo STRAH-nee-tsoo

A few helpful notes:

• dj in dodajte is pronounced like a soft dy sound
• č in vlastoručni is like ch in chocolate
• nj in zadnju is like the ny in canyon
• Stress in Croatian is not always marked in normal writing, so learners usually focus first on getting the consonants and vowels clear

Croatian pronunciation is generally quite phonetic, so once you know the sound of each letter combination, it becomes much easier to read aloud.

Yes, and that is a very natural alternative.

But there is a small difference in focus:

• Dodajte vlastoručni potpis na zadnju stranicu.
  = Add a handwritten signature to the last page.
  Focus: adding the signature

• Potpišite zadnju stranicu.
  = Sign the last page.
  Focus: signing the page itself

In practice, both may be used in similar situations, but the original sentence is a bit more explicit and formal, especially in documents and instructions.