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:
PDFTeX does this to keep file sizes down: suppose you have a document that loads figures fig1.pdf and fig2.pdf; both 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.