DTP hanging while saving pdf with notes

Hi. I bought DTP to manage my large pdf collection in multiple databases, indexing them. But while I open a pdf in DTP and make some annotations (highlighting and notes) it hang for some 2-3 minutes while saving the file.
Doing the same in Apple Preview, it hangs for a little while, but not for some minutes, whit the CPU at 90-100%
Is this normal? How can I fix it?