What MySQL Output is
Not a file format at all. It is what the MySQL command line prints when you run a query: a box drawn with plus signs, dashes and pipes, with a line underneath saying how many rows were in the set.
Anyone who has run a query in a terminal and wants the answer in a spreadsheet.
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 MySQL Output to PDF
The rules and the trailing row count are dropped and the pipes become column boundaries. MySQL prints an unset value as the four characters NULL, which arrives here as an empty cell, because that is what NULL means and because a literal string of NULL in a spreadsheet is worse than a blank. Paste it as CSV instead and every row becomes one column full of pipes, which is why this is its own format.
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 MySQL Output to PDF
MySQL records 4 things about a table that PDF has no way to hold.
MySQL knows which of its values are numbers. PDF keeps only their appearance, so nothing downstream can sort or sum them without someone reading them back off the page first.
An empty cell and a missing one are the same thing in PDF, so a null and an empty string in the MySQL both come out blank and can no longer be told apart.
Nothing in PDF records that the first row is a header. It is drawn at the top and looks like one, which is all that is left of the distinction.
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 MySQL if you will need the values again.