PDF pages are measured in inches, not pixels. DPI (dots per inch) decides how many pixels each inch becomes: an A4 page is 8.3 inches wide, so at 150 dpi it becomes 1240 pixels across.
"PDF to JPG" means two different jobs. Pick the wrong one and you'll either get a picture of a whole page when you wanted the product shot on it, or loose photos when you wanted the page.
JPG blends neighbouring pixels to save space: perfect for photos, but it leaves faint smudges around sharp black text. PNG keeps every pixel exact, which is ideal for text and diagrams but gets very large with photos.
Trim the PDF first, shrink the images after, or go the other way entirely.