Take a look at this thread, I have posted with the same question and got an answer from Laserfiche. https://answers.laserfiche.com/questions/64579/pdf-size-is-larger-after-importingexporting-out-of-Laserfiche#64583 

May I ask how many pages are there in the PDF file?

The original PDF might have been a text-only, or a bitonal (black and white) PDF.

What I mean by text-only PDF is that

• The PDF only contains text, without any image;
• The text is stored as text strings (see: http://en.wikipedia.org/wiki/String_(computer_science) ), instead of an image rendering of the same text.

To determine whether a PDF contains any image, specialized tools (intended for software developers) are needed, although some of these tools are free of charge. One example is the O2 Solutions PDFXplorer: http://www.o2sol.com/pdfxplorer/overview.htm

Text-only PDF files take up very little space per page. When the PDF file is imported with the Generate Pages option, each page in the PDF will be converted into an image. This will increase the size per document page significantly.

When importing PDF into Laserfiche, you have the option of:

• Generating Laserfiche pages, which converts each page into an image
• Generating searchable text, without generating Laserfiche pages

There are advantages and drawbacks as to whether to convert PDF into images. This will depend on the customer's usage (including hands-on usage of Laserfiche Client and backend automations). Converting all pages into images will give the maximum flexibility, at the cost of increasing file sizes. Therefore this option is recommended unless it has been carefully evaluated that the second option will not affect the customer's flexibility in using Laserfiche.