First, allow me to offer a "yikes!" No offense intended.
Many of us will offer "buy a Mac" (I did, long ago...) but there's a few other easy steps to take with PDF files. If you're using a "free" PDF reader application - stop. You work in a law office, they can afford a license of Acrobat Pro/DC - Acrobat DC can be purchased independently of the Creative Cloud subscription and there's a 30-day trial to demo to see if it suits you. If your office is on an older version of Acrobat - upgrade, as many earlier features were deprecated as of version 11. DC is much nicer to use than previous versions. Enough of that.
First, purchase an external SSD or quality USB thumb drive that is of sufficient size to contain the PDF files you're using. My Mini Server's 5400 RPM drives are about 1/3 the read or write speeds of an older SSD (with a USB 3.0 interface) drive I use just for this purpose - to hold temporary files - no internal SSD needed, for now, and I could buy a replacement from the stores I have a PO agreement with today.
Get those PDF files off your system drive - the only other bottleneck on your PC that's slower is your network connection. I use an external SSD for my Photoshop, AutoCAD, Solidworks,
and document searches every day - I used to be the "grunt" in the office, and now I'm the owner...
If you're using Acrobat Reader or Acrobat Pro/DC, there's a few other tweaks to be made to really speed up your workflow.
- In either application, turn off a few options/preferences. First, change the setting for "Page Display>Resolution" from the default of your monitor to the System Setting - that lower resolution will not tax your graphics card nearly as much as the optimum default setting. Under that same group, uncheck/disable the "Rendering" options for "Smooth Line Art", "Enhance Thin Lines" (you're performing text searches...), and "Smooth Images".
- Also, in either application, enable the "Search" Add-in, if it is not enabled - enabling that Add-in activated the by-default-not-activated Search back-end included with Acrobat/Reader.
- Also, in either application, disable any/all of the Add-ins not used in your workflow.
- In the "Internet" setting, change the default "56 kbps" setting to "LAN" or which ever speed is appropriate - as silly as it sounds, file previews have (for me) displayed far more rapidly than the really-slow-default setting.
- Last, if all/most of the PDFs you're working with have "text" in them - you're searching for text/text strings, right? - take a bit of time and export them as RTF (Rich Text Format) files - special text files which are far easier to use/manipulate/search than PDF files, and you'd be searching through the file itself rather than the the "text layer/resource" of the PDF file.
Consider an SSD only after making the above tweaks to your workflow and only after considering buying a shiny new Mac.