This directly adds a cm operation to the content stream in question. Thus, you find those values a..f
specified in the PDF specification ISO 32000-1:
a b c d e f cm Modify the current transformation matrix (CTM) by concatenating the specified matrix (see 8.3.2, "Coordinate Spaces"). Although the operands specify a matrix, they shall be written as six separate numbers, not as an array.
(Table 57)
In section 8.3.3 Common Transformations you'll find some basic transformations: translations, scalings, rotation, and skews. You'll want to use a rotation and probably also a translation.
Section 8.3.4 Transformation Matrices then discusses the mathematics of transformation matrices. Especially it shows that a..f
actually denote the transformation described by the matrix
┌ a b 0 ┐
│ c d 0 │
└ e f 1 ┘
operating on the plane of vectors [ x y 1 ]
by right-multiplication.