Many legal teams use endorsed searchable PDFs as their preferred format for producing electronic evidence. I suspect that two of the most common reasons for this may be that PDFs are a format attorneys are very familiar with, and that the productions can be prepared in-house using the tools the firm has.
I am generally not a fan of PDF productions because I think they lack both the advantages of a native production (e.g. maintaining the metadata and functionality of complex electronic files) and the advantages of a TIFF production accompanied by load files (e.g. flexibility and ease of use with legal review platforms). In fact, our experience shows that upon receiving a searchable PDF production, most law firms hire an outside company, or engage their in-house litigation support team to have the documents converted to a TIFF production with load files so that they can be loaded into a legal review platform.