Change instrument without changing the key

If 'voice' is in D maj, then a transposing instrument as Bari Sax is in B maj. That is correct. So the pitch of every note is matching.
You can either change the score to F maj with the same sounding pitch or you use the command 'Transpose' from D maj into F maj.

Here's what's happening:
When you "import" a PDF, the 3rd-party program Audiveris interprets the file and tries to generate code that MuseScore can display. The scanning program doesn't know if you are scanning in a part for a saxophone or for a flute so it defaults to writing to MuseScore in concert pitch. Now, when you change to a transposing instrument, MuseScore automatically then transposes the engraved music to maintain the pitches. The part has been, essentially, transposed twice so you need to transpose it back again after the instrument change,

I don't think we have all the information.
Was your PDF score originally written for a human voice (or piano, etc. - i.e. for a non-transposing instrument) or for Eb Bari Sax?

If the piece was written for Bari Sax, the notes for a non-transposing instrument were generated during import. It was not recognized that it is a transposing instrument. Then do one of the following methods.
First method:

• apply the key of F major.
• change the instrument to Bari Sax. The staff changes from F major to D major. The notes are almost two octaves too high. But that doesn't matter now.
• Select all the measures and move the notes down. Press Ctrl + down arrow key twice, and then press the up arrow key without Ctrl three times.

Second method:

• Select Tools/Transpose from D major to F major.
• Change the instrument to Bari Sax, The notes are now two octaves too high.
• Select all measures and move the notes down two octaves (Ctrl + down arrow key).

Now it should be right - with any method...