Navigate to your installation folder and look for the internal/help or help directory.
Don't forget that the QB64 IDE comes with a robust internal help system. While not a PDF, it offers instant offline keyword lookups while you code. Getting Started with QB64 qb64 manual pdf
The original QBasic was designed for DOS environments. It utilized a 16-bit architecture with significant memory constraints (limited to 64KB segments). Graphical operations were handled through limited built-in commands ( SCREEN , PSET , LINE ) that interfaced directly with VGA hardware. Navigate to your installation folder and look for
' DO WHILE / DO UNTIL x = 1 DO WHILE x <= 5 PRINT x x = x + 1 LOOP qb64 manual pdf