I tried pdfsam 1.0.0 beta1.It's a useful project.But i find it can't
load big file more than 2X.MB.After 30 minutes it still showing "Please wait while reading xxx.pdf...".
  

Is the log window showing an OutOfMemory error?
pdfsam start with standard memory settings (64mb) that should be enough for your document but you can try running pdfsam with more memory, i just loaded a 320mb document running pdfsam with 1gb of memory.
Try the command java -Xmx128m -jar pdfsam-1.0.0-b1.jar
You can also give more memory (till 2048) changing the Xmx value.

I tried -Xmx128m... ,then it works.
But load 70MB pdf,it still not works,so i change -Xmx256m then it works again.

I use WindowsXP SP1、Intel Celeron 2.4G、512MB RAM、JRE1.6.0.3,and
I'm not a programer just a end user.

Yes,the CMD window showing an OutOfMemory error.

So,I must type the command to cange the Xmx value everytime ,or any other way?

p.s. My English is poor,hope u know what i mean.

There's no feature to set this value in pdfsam so yes, you have to type it every time.. you can also write a little bat/sh script to run pdfsam with the Xmx option.