Zunächst lese ich den Artikel, danach speichere ich die Rezension als PDF.

zunächst is an adverb meaning “first,” “initially,” or “to begin with.” It’s slightly more formal than zuerst, but in many contexts you can swap them without changing the meaning.
• zunächst often appears in instructions or formal contexts.
• zuerst is common in spoken language.
• You can also say am Anfang (“at the beginning”) to emphasize the time rather than the sequence.

Yes, you can say dann speichere ich….
• danach literally means “after that,” pointing back to the previous action.
• dann means “then” and is more neutral.
In everyday speech they’re often interchangeable here.

Using als + noun (nominative) expresses the format or role of something.
• als PDF = “in PDF format.”
• in einem PDF would imply “inside a PDF file,” which isn’t the intended meaning.

PDF is an acronym (Portable Document Format).
• German keeps acronyms in uppercase.
• Without an article, there’s no ending to decline, so PDF stays unchanged.

Yes. Both verbs mean “to save (data),” but:
• speichern is standard.
• abspeichern (a separable verb: ich speichere… ab) is common in everyday language, often emphasizing the action of storing/saving completely.

You can turn the sequence into one sentence with nachdem + perfect tense:
Nachdem ich den Artikel gelesen habe, speichere ich die Rezension als PDF.
Here, nachdem introduces a subordinate clause, so the finite verb (habe) goes to the end of that clause.

Both can mean “afterwards,” but:
• danach refers directly back to a previously mentioned action or time point.
• nachher is more general and can stand on its own:
– danach: “First I’ll read, danach speichere ich…”
– nachher: “Willst du nachher Kaffee trinken?” (Do you want to have coffee afterwards?)