There's numerous causes for this type of behavior, such as:
1) The text/number data formatting is stored that way.
2) Conditional formatting is causing it to resize.
3) The font is unsupported by the PDF distiller.
I'd imagine it's the third one, as didot is a pretty specialized font and I believe it's OpenType (Is your font file .TTF or .OTF ?)
If it's opentype, then save as PDF uses glyphs or outlines instead of true fonts on windows, as only TrueType is supported:
I would suggest switching to a more universally compatible font (IE Times or Georgia, italic) as a substitute font. You can set up a duplicate layout specifically for your PDF creation so you don't have to change your front-facing UI at all.
It is quite likely the font is not embedded in the pdf when it is generated by FM. Have you tried producing the pdf by another technique, such as via the print engine, to see if you get a different result?
The font is Didot.ttc
I don't worry too much about Adobe's guess at font appearances but due to Didot's fitting into several slots and its overall look, it stays.
In the future, I'll take you advice to heart and make some PDF's first to see what works and what gets mangled.
Live and learn.
You might want to test to see if a PDF printer (IE primoPDF or cutePDF) works according to your spec, as keywords suggested, it might be a temporary workaround.
FileMaker's save as PDF ( SCRIPT STEP ) creates a PDF file and Adobe interprets the Didot Font into a mix of Didot and something else. Or did it arrive that way?
FileMaker's Print/PDF/Open PDF in Preview, makes a PDF file and Adobe interprets the Didot font correctly.
FileMaker's Print/Save as PDF, makes a PDF file which is also correctly rendered by Adobe.
The Script step and the Print command do the same thing but differently, one right, one wrong.
I just can't say where the problem actually starts.