"Converted PDF file" - what does it really mean?

I can press Ctrl+P on my keyboard and save this HTML page as PDF. Would it be correct to call such a file a converted PDF file?

No. You should not do this because of possible ambiguity. Because of the frequency of "convert [something] [in]to X" as opposed to relative rarity of "convert [something] from X", the listener will assume the "into" version, and this is not what you want.

You will have to say "the new PDF file."

Long answer:

Past participles as adjectives generally take on a passive function:

The washed car = The car that has been washed. = The indicated/specific car that has undergone the action of washing and must now already be in its new state.

The fed dog = the dog that has been fed. = The indicated/specific dog that has undergone the action of washing and must now already be in its new state.

The converted document = The indicated/specific document that has been converted. The indicated/specific document that has undergone the action of conversion and must now already be in its new state.

Note the past tense of the participle: the action has taken place in the past, thus the indicated noun is currently in the new state.

To convert is ambitransitive but carries with it a mandatory adverbial complement ([in]to X, or from X) that can be explicit or implicit.

A: Where is he living? B: In the house. A: There is no house on his land - there is only a shed. B: He converted the shed. = He converted the shed into a house.

"He converted the house from the shed" is not particularly idiomatic - it sounds awkward - we would say "He made the house from the shed."

To convert [in]to and to convert from are not phrasal verbs because [in]to and from are true prepositions. As such, the preposition cannot be expressed in the passive.

*this is a converted [in]to PDF file./ *this is a converted from PDF file.

Thus a converted PDF file = a PDF file that has been converted [in]to another format - Thus a converted PDF document would have originally been a PDF file that is now in another format, e.g. a .docx format.

We can see this in

"By flattening the head of a large nail, he converted that nail into a screwdriver. He then used the converted nail to unscrew the box."

The alternative could grammatically be a PDF file that has been converted, from another format, but this is redundant in your context, as it is obvious that the file is in the PDF format and does not require a description - it speaks for itself.

*"By flattening the head of a large nail, he converted that nail into a screwdriver. He then used the converted screwdriver to unscrew the box."

and

A: "How did you open the box?" B: *"By converting a screwdriver from a nail".