Microsoft Office previously included an equation editor (Equation Editor 3.0) created by Design Science. This was deprecated to only read (and not edit) expressions in Office 2007, then removed entirely from the Microsoft Office suite in January 2018 because of security problems. (See this help document for background.) When the plugin was removed, the font "MT Extra" was also removed from Windows. The font was later reinstalled through regularly distributed patches.
The lack of the required plugin means that Microsoft Office applications cannot handle the old equations, and may behave strangely. PowerPoint in particular has been observed to hang or crash when attempting to display these equations. Word may show a picture of the equation. Historical documents (e.g. presentations, papers or theses from before 2007) will exhibit this problem. If the "MT Extra" font was not reinstalled, the equations may not display.
Within Office 2019 and in Office 365 , have a conversion utility is available to change Equation Editor 3.0 expressions to the current version.
For older versions of Office, re-entering the equations in the current Microsoft Equation Editor is the preferred method of solving the problem, as this method results in documents that are portable. Installations of Word or PowerPoint elsewhere will able to display and edit the equations. This may be tedious for some documents. If the equations do not need to be edited, copying them as pictures may be sufficient.
The Impress presentation tool in OpenOffice or LibreOffice will read the equations. Those tools will allow you to copy equations from PowerPoint presentations that exhibit the problems and paste the equations as pictures to presentations in native PowerPoint. OpenOffice and LibreOffice are both available for install through the software center.
The current version of the equation editor produces expressions that some do not find acceptable for print. There are alternatives.
A local copy of MathType (another advanced equation editor from Design Science) could be used but these don't solve the problem long term. Licenses for MathType would have to be purchased separately. There may be problems for collaboration if this is used, as the collaborators will also need the tool MathType to edit edquationsequations.
LaTeX (available in the software center as MikTeX) or OpenOffice/LibreOffice may be used for creating documents instead of Microsoft Office, but those would require that collaborators would also have those tools, though the software is available free of charge.
OpenOffice / LibreOffice also have an equation editor.