A:  ReadLn will accept an array [0..something] of Char as buffer to put the read characters in and it will make a proper zero-terminated char out of them. The only limitation is this: the compiler needs to be able to figure out the size of the buffer at compile time, which makes the use of a variable declared as PChar and allocated at run-time impossible.

Workaround:

 Type
   TLine = Array [0..1024] of Char; {use longest line you may encounter here}
   PLine = ^TLine;

 Var
   pBuf: PLine;
 ...
   New( pBuf );

 ...
   ReadLn( F, pBuf^ );

To pass pBuf to functions that take a parameter of type Pchar, use a typecast like PChar( pBuf ).

Note:  you can use a variable declared as of type TLine or an equivalent array of char directly, of course, but I tend to allocate anything larger than 4 bytes on the heap...