For very large and complex PDF files this will help performance slightly, since `EvaluatorPreprocessor.read` is called a lot during parsing in the worker.
This patch was tested using the PDF file from issue 2618, i.e. http://bugzilla-attachments.gnome.org/attachment.cgi?id=226471, using the following manifest file:
```
[
{ "id": "issue2618",
"file": "../web/pdfs/issue2618.pdf",
"md5": "",
"rounds": 200,
"type": "eq"
}
]
```
This gave the following results when comparing this patch against the `master` branch:
```
-- Grouped By browser, stat --
browser | stat | Count | Baseline(ms) | Current(ms) | +/- | % | Result(P<.05)
------- | ------------ | ----- | ------------ | ----------- | --- | ----- | -------------
Firefox | Overall | 200 | 3402 | 3358 | -43 | -1.28 | faster
Firefox | Page Request | 200 | 1 | 1 | 0 | 26.71 |
Firefox | Rendering | 200 | 3401 | 3357 | -44 | -1.28 | faster
```
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