Some customer are unable to download

I've been trying to figure this out now all morning. I've got PDF downloads, they're in the download folder, the attributes are set correctly, the download button shows up in customer's account after payment. All seems well. But when they click on the download button, they get an error message (one got an IE "cannot display this webpage" error, another got an error that says "c:\documen~/Theresa\Locals~1\Temp\ti26u8xs.pdf could not be saved, because the source file could not be read" and several others just emailed me to say they couldn't get the download to work but didn't elaborate and haven't replied to my request for more info)

I have placed several orders myself over the past month using paypal and credit cards and have been able to download successfully each time.

I'm running v1.3.7 on linux, it's set to download by redirect (I tried switching that to false, but it didn't make a difference, customer still couldn't download) Paypal IPN is configured correctly and working well. The PDF files are in the download folder with chmod 644, pub folder is chmod 777. All of the files in the download manager (in admin) have green "valid filename" buttons.

I'm so frustrated, can anyone figure out what is going on here? The main reason I switched to zen was for the download capabilities, and right now I want to pull every hair out of my head.

The store is http://www.littleturtleknits.com/store/ if anyone wants to go in and do some poking around. TIA for any help you can offer!!!

adding more info that might or might not be helpful:
v 1.3.7 is a clean install, on linux server, worked for me during testing and also now works for me live (but apparently I'm the only one it works for, LOL!) There are no spaces or special characters in the file names (it's all numbers between 3 and 5 digits long) they are all PDFs files, no zip files. Largest file is 493KB, smallest is 67 KB. There have been no server changes or hosting changes recently.

anyone? I'm desperate here :( Apparently, I'm the only one able to actually get the downloads to work - none of my customers today who have tried have been successful. One of them sent me this screenshot of what happens when she tries: Screenshot

Hi,

That error... does it occur after the download is completed or before it starts?
After - could be a PDF file error.

You might set up a couple of FREE PDF's for people on the forum to try to download

it happens before - they click the link, and immediately get that error.

I just had one customer email me that had a problem earlier today and she said she got it to download. I haven't been able to replicate the error on my end, so I'm thinking it's got to be something with the end user. She said that she messed around with her spam filters and popup blocker, so she thought it had something to do with either of those. the plot thickens...