if all links begin with <http:// and end with >
try this
set theSource to "Lorem ipsum dolor sit amet, consectetuer adipiscing elit. Proin eleifend ipsum eget nunc.
<http://68.146.xxx.xx/pickup/8884_HeritageHill.pdf>http://68.146.xxx.xx/pickup/8884_HeritageHill.pdf
Morbi mattis velit vel justo scelerisque ullamcorper. Nam tincidunt enim vel nisl. Aliquam nisl
<http://68.146.xxx.xx/pickup/8885_ChaosCity.jpg>http://68.146.xxx.xx/pickup/8885_ChaosCity.jpg
consectetuer faucibus, sem leo dignissim eros, at rhoncus lorem turpis in massa"
set links to {}
set {TID, text item delimiters} to {text item delimiters, "<http://"}
set theSource to text items of theSource
if (count theSource) > 1 then
set text item delimiters to ">"
repeat with i from 2 to count text items of theSource
set end of links to "http://" & text item 1 of item i of theSource
end repeat
set text item delimiters to "/"
repeat with i in links
set fName to last text item of i
if {text -3 thru -1 of fName} is in {"jpg", "pdf", "gif", "zip"} then
do shell script "curl " & quoted form of i & " >" & quoted form of (POSIX path of (path to desktop) & fName)
end if
end repeat
end if
set text item delimiters to TID