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Is the PDF score a scanning of a printed score (= a bitmap graphic)?

Or is the PDF score created from a notation program (= a vector graphic)?

If the PDF score is a bitmap graphic, then you need to use Music OCR software to interpret the “dots”.

Like e. g. SmartScore.

If you have ever used OCR software, then you know that there will be “read errors” - which you will have to correct (= consumes time).

If the PDF score is a vector graphic, then you can use PDFtoMusic Pro.

This will work better than optical scanning programs - because PDFtoMusic Pro knows things that an OCR program can't know.

For instance, PDFtoMusic Pro knows
- that staff lines really are drawn as lines,
- that a notehead is a musical font character, rather than trying to determine a notehead from a bunch of dark pixels on a page.
This is much more accurate than optical scanning for PDFs created by a notation program. SmartScore and other optical scanners have to figure out where staff lines and noteheads are from the dots on a page, which is more error-prone.

The flip side is that PDFtoMusic Pro cannot handle optical scans from a scanner at all. For those you need an OCR program - like SmartScore.

... copy notes from a PDF score, ...

I know of no way to do that directly. An additional software application must be used. Smart Score Pro turns your PDF into a tiff file and then SS Pro turns that into an XML file. You can import the XML file into Finale, copy and paste, into your Finale document. As mentioned above if you have good copy, you will get good results. If your copy is old, yellowed, perhaps a tear or two, the resulting errors will be more work to correct than its worth.

Peter gave you half the answer (an actual pdf doc has font info) and Ernest gave you the other half (if the pdf is a picture of the file, it will not have font information). Nearly all pdf files found on the internet are the later kind. Scans cannot import font information. 

SmartScore 64 Pro is available for half price to Finale owners. If you're doing orchestral scores, it's the only version to consider.

https://www.musitek.com/store/Fin2SS64.html 

Besides being able to export directly to Finale, this allows you to edit your scan before export—normally, you'll need to—but it's still a lot less work than entering every line from scratch.

Is there a simple way of knowing whether a PDF file is a bitmap or vector graphic ?

Only if you know how it was generated. Otherwise, the only way I know is if an app like PDFtoMusic Pro cannot read it. Myriad offers a trial version; it will try to read one page free:

https://www.myriad-online.com/en/products/pdftomusicpro.htm 

I don’t bother with it since anyone I know will send me MusicXML and we will have discussed which version beforehand. If I download anything from the MuseScore site, I will export the .xml myself.

Most people who upload to the internet sharing sites will do bitmapped graphics files to keep others from accessing their work directly. With OCR scanners, it doesn’t matter as they only see pictures. SmartScore 64 Pro is not perfect but it does save a lot of time. 

Is there a simple way of knowing whether a PDF file is a bitmap or vector graphic ?

Try printing it with one of the PDF print drivers like Cute or DoPDF. That should give you a PDF that Smartscore 64 can read.

The question is about importing scores, not printing them.

However that would be a way to get that task accomplished. Would it not?

… Is there a simple way of knowing whether a PDF file is a bitmap or vector graphic ? …

Something to try:

While viewing the PDF file in a PDF viewer (like e. g. Adobe Reader), try zooming in to a high zoom percentage.

If you get jaggies, then the graphic is a bitmap graphic.

If you get clear, sharp edges, no matter how much you zoom, then the graphic is most likely a vector graphic.

Also, the file size can give a clue.

A vector graphic has a small file size, while a bitmap graphic can be huge.

That is a good trick. How ironic that the "best" PDF for simple inputting into Finale is the one with the smallest file size !