[nhedgehog]

Or you can use another reader software, f.i. KoReader or Coolreader, the later allows tinkering with the dictionary window via the skin file but doesn't open pdf's.
PDF size does not say anything about the readability. I've seen pdf's of only 10 pages on which 2 pages where full of little vector graphics, demanding the most of any pdf viewer and other 50mb pdf files with hundreds of pages, beeing very easy to view on my pocketbook. All Pocketbooks are using comparable software (if compared in the same firmware number range).
FYI this is what you can change in Coolreader regarding the dictionary:

<window id="dict" size="100%, 44%">
<title>
<size minvalue="0,100" maxvalue="100%,100"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="40" bold="true" italic="false" valign="center" halign="left"/>
<background color="#FFFFFF"/>
<background color="#000000" pos="0, 0" size="100%,1"/>
<background color="#000000" pos="0,-1" size="100%,1"/>
<background color="#000000" pos="0,0" size="1,100%"/>
<background color="#000000" pos="-1,0" size="1,100%"/>
<border widths="8,4,4,4"/>
</title>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="35" bold="true" italic="false" valign="center" halign="center"/>
<client>
<background color="#FFFFFF"/>
<border widths="8,8,8,8"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="34" bold="false" italic="false" valign="center" halign="center"/>
</client>
<status>
<border widths="0,0,0,0"/>
<size minvalue="0,1" maxvalue="0,0"/>
<background color="#000000"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="1"/>
</status>
</window>


<window id="dict-rotated" base="#dict"/>
<menu id="dict-list" min-item-count="10" max-item-count="10" show-shortcuts="false" fullscreen="false" >
<title>
<size minvalue="0,100" maxvalue="100%,100"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans Condensed" size="39" bold="true" italic="false" valign="center" halign="left"/>
<background color="#FFFFFF"/>
<background color="#000000" pos="0, 0" size="100%,1"/>
<background color="#000000" pos="0,-1" size="100%,1"/>
<background color="#000000" pos="0,0" size="1,100%"/>
<background color="#000000" pos="-1,0" size="1,100%"/>
<border widths="4,4,4,4"/>
</title>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="39" bold="true" italic="false" valign="center" halign="left"/>
<background color="#FFFFFF"/>
<border widths="4,0,4,0"/>
<client>
<background color="#FFFFFF" />
</client>
<value>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="35" bold="false" italic="false" valign="center" halign="left"/>
</value>
<item>
<border widths="0,0,0,0"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="35" bold="true" italic="false" valign="center" halign="left"/>
</item>
<selitem>
<border widths="0,0,0,0"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="35" bold="true" italic="false" valign="center" halign="left"/>
</selitem>
<status>
<border widths="0,0,0,0"/>
<size minvalue="0,1" maxvalue="0,0"/>
<background color="#000000"/>
<text color="#000000" face="Liberation Sans, DejaVu Sans" size="1"/>
</status>
</menu>
<menu id="dict-list-rotated" base="#dict-list" min-item-count="7" max-item-count="7"/>