In the following, is "viel" a pronoun or adverb?
Viel zu publizieren ist der Schlüssel zur Hochschulkarriere.
With viel you have, just like manch and wenig, hit a class of words that somewhat escapes a clear classification.
Simplyfying your example to a plain
Er publiziert viel.
Makes classification a bit easier, but not in any way clearer:
It could be an adjective - arguments for that may be that it has a comparative (mehr) and a superlative (am meisten). It can also be flexed like an adjective, can, however, not be used in a predicative way ("er ist viel") like you can with typical adjectives .
Er publiziert mehr [als Hans]
It could be an indefinite pronoun or numeral - compare to
Er publiziert nichts
Or it could be (understood in a temporary sense) an adverb in its original (not the "catch-all") sense, comparing to
Er publiziert oft
You could even assume classification as a Gradpartikel such as
Er publiziert wenig
Each of the above classifications has some validity and some good arguments for, but unfortunately, none of them fit 100%. In the typical "that seems to be complicated, so it must be an adverb" way of evading endless discussions, most grammars and dictionaries would assume "viel" in your example as an adverb, really good ones would mention the controversy.
"Viel" (much) is always an Adverb. It specifies a property of a Verb to which it is correlated:
publizieren - viel publizieren
laufen - viel laufen
etc..
What might be a problem understanding the sentence is the non-neutral word order used. German sentences have a "neutral" word-order where nothing in particular is emphasized. Deviate from this order and you put emphasis on a certain part (usually the one moved to the beginning or the end).
Here the neutral order would be:
Der Schlüssel zur Hochschulkarriere ist, viel zu publizieren.
"Der Schlüssel" is the Subjekt (subject) here, "zur Hochschulkarriere" is a specification of the properties of this specific key. The key to what? The key to a unversity career.
The Prädikat (predicate) of the sentence is "ist". German isn't a SPO-, but a V2-language. Still, the "neutral" order is SPO. Here, "viel zu publizieren" is moved up front to put emphasis onto it. You can do something similar in English, but it is a bit more complicated:
Publicize a lot, this is the key to a successful university career.
Nota bene, there is a vast difference between "viel" (much) and "vieles" (a lot [of ...]). This in fact can act as a pronoun (i.e. "Vieles wurde gesagt." but in contrast: "Es wurde viel gesagt.") This is how words like "nichts", "manches", "alles", etc. work: they can be interpreted as (for lack of a better word) words-made-pronoun.
Viel zu publizieren ist der ...
It is a Freudian Slip is what it is.
Viel publizieren ist ...
a Determiner Phrase that predicates the argument. As the subject, Publizieren has to be a noun.
"Common kinds of determiners include definite and indefinite articles (the, a), demonstratives (this, that), possessive determiners (my, their), cardinal numerals (one, two), quantifiers (many, both), distributive determiners (each, every), and interrogative determiners (which, what).
Whether participles are verbs or adjectives is controversial.
" In linguistics, a participle (from Latin participium 'a sharing, partaking'; abbr. ptcp) is a nonfinite verb form that has some of the characteristics and functions of both verbs and adjectives
The zu-Partizip is therefore not a noun.
Therefore, viel-zu has to be a noun. This is in line with the derivational morphology of other nouns such as Blitz, Holz, Kraftprotz, Putz. Indeed, blitze-blank, putz-munter and maybe ratze-puse-leer for example are typical adverb+adjective compounds. The u-vowel is best attributed to the w-verbs, accounting for the verbal haed of the subordinate verbal phrase. Incidently, this accounts for Germanic innovative u-stems.