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 JPEG is
A lossy image format designed for photographs, where losing a little detail in a gradient is invisible.
Places that will not accept a PNG upload.
What changes when you convert PDF to JPEG
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.
The same render as PNG, saved as JPEG. It is the worse choice for a table and it is worth knowing why: JPEG compresses by discarding detail around sharp edges, and a table is nothing but sharp edges. Letters get a visible halo and hairline rules blur. It also has no transparency, so a background colour is always painted rather than left clear.
What carries over from PDF to JPEG
Both formats show a table rather than hold one, so this changes how the page looks and not what can be read back out of it.
Neither format holds the table as data. PDF describes a page as glyphs at coordinates, so a table in a PDF is a visual arrangement rather than a structure. JPEG compresses lossily and softens text edges, which is why a table saved as JPEG reads worse than the same table as PNG. So this is one rendering of the table becoming another, and the values are equally out of reach on both sides of it.