You have a PDF figure, which you want to use in your pdfLaTeX document. When you compile the document, pdfTeX complains about “missing glyphs”, and some (or all) of the labelling text or symbols in the original figure is no longer visible.
What has happened is:
fig.pdf) has a font
font.pfbembedded in it.
font.pfbon disc, and loads that in place of the copy in
fig.pdfhas glyphs that aren't in
font.pfbon disc, so that you get errors while compiling and you see that characters are missing when you view the output. (pdfTeX can't know that the fonts are different, since they have the same name.)
Which is all very undesirable.
pdfTeX does this to keep file sizes down: suppose you have a
document that loads figures
of those use font
font.pfb. If pdfTeX takes no action,
there will be two copies of
font.pfb in the output.
(If your document also uses the font, there could be three copies.)
A real case is the URW font
NimbusRomNo9L-Regu (a clone
of Times Roman), which is available in a version with Cyrillic
letters, while the version in TeX distributions doesn't have those
letters. Both versions, as distributed, have the same name.
The simple (“quick and dirty”) solution is to add the command
to the preamble of your document.
The “real” solution is that one or other font should be renamed. In either case, this would require that you reconfigure some program's (TeX's or your drawing package's) font tables - inevitably a tiresome job.