You’ll have to be more specific with your operating situation. PowerShell can only do Get-Content on plaintext files, which your documents probably aren’t. Handling other document formats requires more complicated methods, and it’s different for each document format that you want to handle. Also, writing new information into them will be more complicated than getting information out of them.
For instance, this blog post from the Scripting Guy describes a method for importing a .docx file as an object and then finding specific words within the file.
This forum discussion is about finding text in a .pdf document, but it relies on the now-deprecated itextsharp. You can probably apply the same method using the new version, iText7, but depending on your usage it may not be legal to use it for free. Unclear whether this can handle .ps documents in addition to .pdf
If you need to handle other document types, like .odt, that’s another specific solution that you’ll have to find.