Without seeing a sample file and knowing more about your configuration (i.e. OS and version, Acrobat version, etc.), it is next to impossible to fully diagnose the problem.
What we can tell you is that the likelihood of achieving good / usable results really depends on:
(1) How the PDF file was created — tagging. If it was created with tagging it is easier for the export function to second-guess what the contents should appear like in a PowerPoint file.
(2) How the PDF file was created — transparency flattening. If transparency in an original document was flattened in the process of creating a PDF file (such as PDF 1.3-based settings), all bets are off since some (or much) of the original content may have been outlined or rasterized in PDF creation.
(3) Source of the PDF file. If the PDF file was created from a PowerPoint presentation, then the contents are more likely to be recognized in terms of what should be in a PowerPoint file. Furthermore, for example, PDF files created from InDesign, Illustrator, CorelDRAW, etc. all use graphic constructs that simply don't exist in PowerPoint (or other Office document formats for that matter) and don't translate well to the limited graphics model of Microsoft Office.
(4) Fonts. Font is a four letter word beginning with an F! If you created the PDF file with a tool that doesn't properly encode the fonts or uses some oddball, non-standard encoding, the resultant exported PowerPoint file may show total gibberish in terms of text. Furthermore, even if the fonts are encoded correctly and embedded in the PDF file, if those fonts simply are not installed on the system on which you open the PowerPoint file that results from the export, PowerPoint will substitute fonts will some of the symptoms you describe.
Without seeing your file, I would hazard a guess that at least two if not more of the above-factors are involved.
Post a sample someplace and we might be able to further assist you.