The graphics we want are on page 50, but there are six prefatory pages labelled i through vi. So we will start by loading page 56 into memory:
$url = "http://ediss.sub.uni-hamburg.de/volltexte/2004/1133/pdf/dissertation.pdf";
$page50 = Import[$url, {"Pages", 56}] // First

We used First
because importing PDF "Pages"
returns of list of pages, in this case of one element.
The imported page is a Graphics
expression:
$page50 // Head
(* Graphics *)
We can extract individual graphical elements from that expression. For example, here is the chapter title:
$page50[[1, 2]] // Graphics

It does not take much experimentation to locate our plots:
$page50[[1, 3;;30]] // Graphics

... or to extract just the first curve:
$curve1 = $page50[[1, 3]];
$curve1 // Graphics

The curve points can be extracted from this curve expression:
$curve1
(* Style[{JoinedCurve[{{{0, 2, 0}, {0, 1, 0}, ...}},
{{{138.816, 709.0320000000002}, {139.023, 707.23},
{139.20000000000002, 701.972}, ...}}, CurveClosed -> {0}]},
JoinForm[{"Miter", 10.}], Thickness[0.0014893882352941176],
RGBColor[0.47100000000000003, 0.47100000000000003, 1., 1.]]
*)
$curve1[[1, 1, 2, 1]]
(* {{138.816, 709.032}, {139.023, 707.23}, ..., {286.514, 655.299}} *)
$curve1[[1, 1, 2, 1]] // ListLinePlot

We need to rescale the points to the original axes:
$points1 //
Transpose //
Query[{1 -> (100 Rescale[#]&), 2 -> (2 Rescale[#] - 1 &)}] //
Transpose //
ListLinePlot

In the general case, rescaling can be more challenging. See (85329) for a more general treatment.
We have successfully recovered the original data points of the first curve and replotted them using Mathematica.
For completeness, here are the "addresses" of all of the individual curves:
$page50[[1, {3, 4, 10, 11, 17, 18, 24, 25}]] // Map[Graphics] // Column

copyCurve
published here: mathematica.stackexchange.com/questions/44355/…. It enables you to capture the points from the PDF plot as a list and to use them then in Mma. In your case of a highly oscillating functions it will require some work, to catch the curves in enough details. $\endgroup$