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 BBCode is
BBCode uses square-bracket tags. Table support is not universal, but table, tr, th and td are widely implemented.
phpBB, vBulletin and forum software descended from them.
What changes when you convert PDF to BBCode
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.
Check that your forum has table tags enabled. Many boards ship with them switched off.
What carries over from PDF to BBCode
PDF records 2 things about a table that BBCode has no way to hold.
A line break inside a cell ends the row in BBCode, so line breaks are replaced rather than carried through.
Anything visual in the PDF - weight, alignment, colour, column widths - has no counterpart in BBCode and is dropped. The values are what survives.