How to download attachments in bulk if the size exceeds 100 MB. Need to download documents for 3-4 years

Hello, gauri. 

You can download attachments in bulk by exporting them. However, you can download attachments to 300 rows. Also, exporting attachments on 1 page only is not possible. 

I understand the importance of downloading attachments on a single page. While the option is unavailable, I recommend sending feedback directly to our product engineers to let them know about the needed feature.  Rest assured that they are constantly working to improve the product to deliver the best possible experiences for all QuickBooks users.

Here's how:

1. Go to the Gear icon at the top, then feedback.
2. Enter your comments or product suggestions.
3. Hit Next. 

Here's an article you can open for more tips while managing attachments in QBO: Add or delete attachments in QuickBooks Online. 

Please feel free to leave a comment below if you have follow-up concerns. Have a good one!