Powershell - Need help putting grouping files into Hash Table based on partial matching names

Question

I'm trying to put PDF files into a Hash Table Group based on the file names which use different naming conventions.

The first set of files follows the following format:

0343453 - SHORT NAME.pdf
0325423 - MEDIUM NAME.pdf
0323546 - A VERY LONG NAM.pdf

The second set of files follows the following format:

SHORT NAME % BGHD.pdf
SHORT NAME % HDJS.pdf
MEDIUM NAME % 123432.pdf
MEDIUM NAME % 453222.pdf
A VERY LONG NAME WITH MORE THAN 15 % HGHB.pdf
A VERY LONG NAME WITH MORE THAN 15 % FR54.pdf

In the first set of files the second part (after the '-') is limited to 15 characters, but it could be shorter, and this matches the first part of the second group of files up to 15 characters.

My code is below, which has the key as being the test after the '-' in the first group of files. The first group is getting put into the Hash Table correctly. I am getting this error for the second group of files: Method invocation failed because [System.IO.FileInfo] does not contain a method named 'LastIndexOf'.

cd C:\test\group1
$ch = Get-ChildItem *.pdf 

cd C:\Test\group2
$in = Get-ChildItem *.pdf

$merged = @{}


foreach ( $i in $ch) {
$merged += @{$i.BaseName.Substring(10) = $i}
}



foreach ( $j in $in) {
if (($j.BaseName.Substring(0,$j.LastIndexOf("%")).trim("%").length) -le 15) {
$merged += $j.BaseName.Substring(0,$j.LastIndexOf("%")).trim("%") = $j
}
else {
$merged += $j.BaseName.Substring(0,$j.LastIndexOf("%")).trim("%").SubString(0,15) = $j}
}