What XML is
XML marks up data with nested named tags and optional attributes. It is verbose, strict about being well-formed, and can carry a schema that says what shape a document must have.
Enterprise integrations, government data feeds, RSS, SOAP services and older document formats.
What PDF is
PDF describes a page: glyphs at coordinates, not rows and columns. A table in a PDF is a visual arrangement, not a data structure.
Reports, invoices, statements and anything sent to be read rather than processed.
What changes when you convert XML to PDF
The first repeating element is treated as the rows. Attributes are read alongside child elements and prefixed so they do not collide with a same-named tag. Nesting one level deep becomes dotted column names; deeper structures are kept as text.
The table is drawn into a paginated document with the header repeated on every page, so a long table stays readable past page one. Page size, orientation and font size are all adjustable.
What carries over from XML to PDF
XML records 3 things about a table that PDF has no way to hold.
An empty cell and a missing one are the same thing in PDF, so a null and an empty string in the XML both come out blank and can no longer be told apart.
XML can hold a value that is itself a list or an object. PDF has only flat cells, so nested values are flattened into one cell rather than being spread across columns.
PDF is the end of the line for this data. Reading the table back out of it is a separate and much harder problem than writing it, so keep the XML if you will need the values again.