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 Plain Text is
Plain text here means columns lined up with spaces, the way a command line tool prints a table.
Terminal output, log files, fixed-width exports from older systems.
What changes when you convert PDF to Plain Text
Every character in a text-based PDF carries its position on the page. Characters sharing a baseline are one row. Columns are found from the vertical strips no character ever occupies, which is why extraction works cleanly on a document laid out with spacing and badly on one laid out with ruled lines. A header repeated at the top of each page is detected and dropped rather than landing in the middle of the data.
Columns are padded to the widest value. Width is counted in display columns rather than characters, so a table containing Chinese, Japanese or Korean text still lines up: those glyphs occupy two columns in a monospace font.
What carries over from PDF to Plain Text
PDF records 2 things about a table that plain text has no way to hold.
A line break inside a cell ends the row in plain text, so line breaks are replaced rather than carried through.
Anything visual in the PDF - weight, alignment, colour, column widths - has no counterpart in plain text and is dropped. The values are what survives.
This is the direction that recovers structure: PDF has no table in it to read, only an arrangement that looks like one, so the rows and columns are inferred rather than read.