You can do it either way.
Recommendation: add the data via read-only tabs when your app creates the DocuSign envelopes. This will be the fastest to implement and can provide good results.
More about the options:
Add the dynamic data to the DocuSign envelope
You can add text tabs (also known as fields), or other types of tabs, to the document when your application creates the envelope. You can also use a template that includes the document and positions the tabs.
When you create the envelope, your application sets the values of the tabs. You can set the tabs to be read-only so the signers can't change the values. Or for some use cases, you can set the values and then the signer can update the values as appropriate. (And your app can later process the updated values.)
You can use multi-line text tabs to include lots of updated information if you wish.
Benefit: your business people can update the PDF, and if they're given guidelines and do it carefully, then the PDF updates can be done without involving the application developer.
Downside
While you can set the tab's font and font size, it can be tricky to get the added text to seamlessly look like it is part of the PDF. How much this matters to you depends on the use case.
Generate custom PDFs for each envelope
You can also do this. Benefit is that the dynamic text will be a part of the PDF itself and presumably will exactly match the document's font, size, placement, etc.
Downside
Generating custom PDFs is much easier these days, but it still tends to be much more work for the programmer than the API calls used to add text tabs to the document.
If you programmatically generate the PDF then any changes to the original PDF will probably require the programmer to be involved in the update.
Generate an HTML doc on the fly
If you really want everything to look exactly right, an alternative to generating custom PDFs may be to generate custom HTML documents for DocuSign to turn into PDFs. This also enables you to take maximum advantage of the DocuSign Responsive Signing and Smart Sections features.
But, like the custom PDF solution, it becomes harder to update the document.