What IIF is
IIF is QuickBooks Desktop's import and export format: tab-separated, with several tables interleaved in one file. A line starting with an exclamation mark defines the columns for a record type, and the data lines that follow begin with that type's keyword.
QuickBooks Desktop users, and the bookkeepers who have to move data into and out of it.
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 IIF to PDF
The largest table in the file is used, which is almost always the transactions. A typical export holds TRNS rows for the transactions and SPL rows for their split lines; those are two tables, not one, and combining them would produce a column layout that fits neither.
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 IIF to PDF
IIF records 4 things about a table that PDF has no way to hold.
IIF 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.
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.
IIF can be read one record at a time. PDF has to be complete before it means anything, so the whole table is held in memory while it is written and a file that gets cut off part way through is not partly usable - it is unusable.
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 IIF if you will need the values again.