This is a workaround I figured out today from reading a thread asking the same thing for the k5...(didn't really explain as well) here:
https://www.mobileread.com/forums/sho...d.php?t=211046
I am now able to download PDF files from google searches and goes straight to /mnt/us/documents . woots
you need to be jailbroken. have terminal access or usbnetwork.
I did this with myts and mntroot.
This will not step through everything, you should be able to fill in, or else you should be doing this probably.

this is not meant to be followed as a step by step guide, its hinting at what to do, and the methods may be subjective based on users prefered methods.
1. open a terminal, or ssh, and make a backup of /usr/bin/browserd
2. create a second copy to open in a hex editor.
3. search the file for "mobipocket"(this is the one i edited)
4. you should see a MIME/HTTP type header "application/x-mobipocket-ebook"
what I did was kept "application/x-m" there and replaced the rest of the string, "obipocket-ebook" with 00 hex, which in most hex editors should be represented by a "."
5. save.
6. remove the old browserd, replace it with the new one.
7. reboot.
heres what i did in myts
#: cp /usr/bin/browserd/ /mnt/us/browserd.bak
#: cp /usr/bin/browserd/ /mnt/us/
did the previously mentioned steps(1-7), editting the /mnt/us/browserd that was created and went back to myts, assuming the mount was lost
#: mntroot rw
#: rm /usr/bin/browserd/
#: cp /mnt/us/browserd /usr/bin/
reboot.
if you messed up you have the .bak .